back pain, stress at work, medically unexplained fatigue, and (4) _____ commonly colds.

Knowledge: Determiners / pronouns

A. almost + verb / adjective / noun (adv): almost

B. most: most, almost => most + long adjective/long adverb: … most

C. mostly (adv): mostly

D. at most = not more than: at most

Because “commonly” is an adverb, we don’t use the previous adverb anymore, we need to use the superlative.

… but medicine has little to offer – back pain, stress at work, medically unexplained fatigue, and (4) most common colds.

=> …but drugs are of little help – back pain, work stress, medically unexplained fatigue and most commonly a cold.