The correct option is A longest
Adverbs modify a verb, an adjective and sometimes another adverb in a sentence.
Some adverbs, like adjectives, have three degrees of comparison.
If the adverb is of one syllable, we form the comparative by adding er, and the superlative by adding est, to the positive; as, 'fast-faster-fastest'.
Option A is correct as longest is the superlative degree of long.
Option B is incorrect as long is the positive form
Option C is incorrect as longer is the comparative form of long.
Option D is incorrect because double superlative is an incorrect structure.