Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is the use of a colon within a sentence. In this choice, the colon correctly introduces the following description of how the painting makes use of the entire visual light spectrum by depicting a rainbow-hued swimmer.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a run-on sentence. The main clauses ("Like…spectrum" and "sporting…water") are fused without punctuation and/or a conjunction.
Choice C
Choice C is incorrect because it results in a run-on sentence. The main clauses ("Like…spectrum" and "while…water") are fused without punctuation. Furthermore, the conjunction "while" fails to indicate that what follows is a description of how the painting makes use of the entire visual light spectrum.
Choice D
Choice D is incorrect because it results in a logically confusing sentence. The conjunction "while," which suggests that what follows is occurring at the same time as or despite what came before, fails to indicate that what follows is a description of how the painting makes use of the entire visual light spectrum.