Answer:
A
Step-by-step explanation:
iii cannot be correct because there is no credibility being alluded to by the author.
ii cannot be correct because there is no semblance of similar grammatical structure in the first or latter parts of the sentence.
i is the only correct answer because the comparison of the author's reasoning being "as straight and clear" to a "ray of light" makes the author's reasoning appear as or in contrast to something else.