Step 1: Understanding the Concept:
This question tests the correct use of comparative and superlative adjectives ("more" vs. "most") and adverbs vs. prepositional phrases ("certainly" vs. "for certain").
Step 2: Detailed Explanation:
More vs. Most: The comparative form ("more perplexing") is used when comparing two items. The superlative form ("most perplexing") is used when comparing three or more items. The sentence begins with "Of the numerous decisions," which implies there are many (three or more) decisions being considered. Therefore, the superlative "most perplexing" should be used, not the comparative "more perplexing." This eliminates (A), (B), and (E).
Certainly vs. For Certain: "For certain" is a prepositional phrase that is generally considered less formal and less standard in written English than the adverb "certainly." The adverb "certainly" is the better choice to modify the verb "is."
Let's check the remaining options:
(C) ... is certainly the most perplexing. This option correctly uses the superlative "most" and the standard adverb "certainly."
(D) This option restructures the sentence with "it seems," which is weaker, although it correctly uses "most."
Between (C) and (D), (C) is more direct, concise, and stylistically superior.
Step 3: Final Answer:
Because the comparison involves "numerous" (more than two) decisions, the superlative "most" is required. The adverb "certainly" is idiomatically correct. Option (C) incorporates both corrections.