The correct option is (C) as safe as.
Positive degree is the base form of the adjective or adverb. It does not show comparison. If we look into Option C, the structure as…asis used to compare things that are of similar proportion. In this case the firstasacts as an adverb modifying the adjective or adverb that goes after it. The secondascan act as a preposition or conjunction. If it is used as a preposition, it will be followed by a noun or pronoun. If it is used as a conjunction, it will be followed by a clause. The structure as…as cannot be used to measure things of unequal proportion. In this case, we have to use comparative + than or superlative+than. This makes as....as a positive degree of comparison. In this particular sentence, the comparison is of similar proportion. Option C is correct.
Option A and B are superlative degree and Option D is comparative degree.
