Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is used to correctly mark the boundary between one sentence ("A ray...works") and another ("Because...image").
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a run-on sentence. The two sentences ("A ray…works" and "Because…image") are fused without punctuation and/or a conjunction.
Choice C
Choice C is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between sentences.
Choice D
Choice D is incorrect because it results in a comma splice. Since the contraction "it’s" creates a main clause, the comma after "single ray" can’t be used in this way to mark the boundary between two main clauses ("it’s...ray" and "all light…image").