In order to be able to answer this question, we first need to understand what comparative and superlative adjectives are. Comparative and superlative adjectives are two-syllable words, like fast, to whose ending will be added an er, or, est, in order to modify a noun or verb and compare and contrast them. In the comparative forms, the ending that is used is er, and these are used when we want to establish a comparisson between two words. For example: "Catalina is faster than Susana". The superlatives, in contrast, are used to establish superiority of one noun, or verb, over all others, or a group, as such: "Catalina is the fastest of her cohort". In this question, we have precisely the use of the word "fast" as an adjective that modifies the noun "elevator". In order for it to become a correct sentence, we need to use the form of the modifier that is correct, and in this case, the correct answer would be A: faster, because in this sentence, the word being used is a comparative adjective form, and its correct ending is -er-.