The correct option is B written
Adverbs modify verbs, adjectives and sometimes other adverbs in a sentence.
In this sentence, the verb 'written' is modified by the adverb 'well'. It shows the manner in which something is written. Hence, Option B is correct.
Option A is incorrect as 'well' is the modifying adverb in this sentence.
Option C is incorrect as adverbs don't modify nouns.
Option D is obviously incorrect.