Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of punctuation in a sentence. In this choice, a colon is correctly used to mark the boundary between one main clause ("Along with...photosynthesis") and another main clause ("as light...oxygen") and to introduce the following explanation of how light intensity affects photosynthesis.
Why the other choices are wrong
Choice A
Choice A is incorrect because when coordinating two longer main clauses such as these, it’s conventional to use a comma before the coordinating conjunction ("and").
Choice B
Choice B is incorrect because it results in a comma splice. Without a coordinating conjunction following it, a comma can’t be used in this way to join two main clauses ("Along with...photosynthesis" and "as light...oxygen").
Choice D
Choice D is incorrect because it results in a run-on sentence. The two main clauses ("Along with...photosynthesis" and "as light...oxygen") are fused without punctuation and/or a conjunction.