The Berlin–Kay formulation in the problem states that the order of color acquisition is: A, B → C → D (from B) → E → F (from D) → H → G (from A, D, or F). Of course, one can exercise discretion and realize that certain colors are more fundamental than others, but let's do it via pure logic.
From rule 3: 5-and-above languages have a unique word for E. Looking at the diagram, the only color that is always unique from Apinaye/Seri and above is "banana", so E = banana.
From rule 5: The last two acquired words are H and G. 7-color languages English/Yakan, compared to the 6-color languages Guambiano/Teribe, developed the additional unique word for "plum", so H = plum. 8-color languages additionally separate "sky" vs. "ocean", so G ∈ { sky, ocean }.
From rule 5: 6-and-below languages merge G with one of A, D, or F. If G is "ocean", then among all 6-color languages (Guambiano/Teribe/Tlapanec), other than "plum", it only ever shares the same name with "crow" and "sky/grass". If G is "sky", then it only ever shares the same name with "snow" and "ocean/grass". It seems safe to assume that A, D, and F map to these three colors.
From rule 4: 6-and-above languages always distinguish all of A, D, and F, but Tlapanec doesn't between sky and grass, we can conclude that G = sky, { A, D, F } = { snow, ocean, grass }.
From rule 2: "Snow" must be either A, B, or C, because by 3 colors it already gets a unique name. Therefore A = snow, { D, F } = { ocean, grass }, { B, C } = { crow, strawberry }.
From rule 1: A and B must be distinguished from the beginning. Bassa does not distinguish "strawberry" from "snow", so B cannot be "strawberry". Therefore B = crow, C = strawberry.
From rule 3: 5-and-above language distinguish B "crow" and D. Tsafiki does not distinguish "crow" and "ocean", so D = grass, F = ocean.
So let's restate Berlin–Kay:
Every language has at least two basic color terms, which distinguish between "snow" and "crow".
If it has at least three, it uses different words for each of "snow", "crow", and "strawberry". If it has only three, the same word is used for both "crow" and "grass".
If it has at least five, it distinguishes between "crow" and "grass", and also has a unique word for "banana".
If it has at least six, it distinguishes between "grass" and "ocean".
A language only has a unique word for "sky" if it also has one for "plum". If it does not have a unique word for "plum", "sky" will share a word with at least one of "snow", "grass", or "ocean".
C2 answers:
Kwerba: 4 colors, so rules 3 and 4 are moot. "snow", "crow", and "strawberry" are distinguished, satisfying rule 2. No unique word for "plum", and "sky" shares with "snow", satisfying rule 5. — YES
Lele: 4 colors, so rules 3 and 4 are moot. "snow", "crow", and "strawberry" are distinguished, satisfying rule 2. No unique word for "plum", and "sky" shares with "grass", satisfying rule 5. — YES
Ocaina: 6 colors, but "grass" and "ocean" are not distinguished, violating rule 4. — NO
Waorani: 4 colors, so rules 3 and 4 are moot. "snow", "crow", and "strawberry" are distinguished, satisfying rule 2. No unique word for "plum", and "sky" shares with "ocean", satisfying rule 5. — YES
Yucuna: exact same case as Ocaina. — NO
Zapotec: 7 colors. Has a unique word for "plum", satisfying rule 5 (the rule doesn't require a unique word for "sky" in the presence of a unique word for "plum"). No other rule requires the distinction between "grass" and "sky". — YES
C3 answers:
a. There are 3 unique words, so only rules 1, 2, and 5 apply. Nothing is said about "banana", so all three options are possible.
b. There are 5 unique words, so rules 1, 2, 3, and 5 apply. Only rule 3 pertains to "grass", stating that it must be distinguished from "crow", and cannot share a word with "banana". So the answer is all except "crow" and "banana": sõa'ro, ya'saro, butiro.
c. There are 6 unique words. Rule 2 requires "strawberry" to be distinguished from "snow" and "crow". Rule 3 requires "banana" to be unique. Because "sky" shares with "ocean", rule 5 is satisfied regardless of whether "plum" is unique or not. So the answer is all except "snow", "crow", and "banana": likañ, minj-kimemb, muk.