A, C, E - Incorrect: The comparison nearly 1,500 years as old as is nonsensical.
D, E - Incorrect: The adverb supposedly might be interpreted to modify the noun Sierra Nevada even though it should modify contemporaries. Of course that would not work either, because contemporaries is a noun here and supposedly is an adverb. Adverbs can only modify verbs, adjectives, and sometimes other adverbs.
B - Correct: The use of the adjective supposed correctly modifies contemporaries. The use of the comparative form older than creates a logical meaning.
B is the correct answer.