Historians have long believed that the invention of the printing press was an instrumental factor for the widespread dissemination of knowledge, accelerating the Renaissance and facilitating the Reformation.
Correct: C
The original sentence uses the phrase 'an instrumental factor for,' which is less idiomatic and more wordy than 'instrumental in.' The adjective 'instrumental' (meaning 'serving as a means; helpful') followed by 'in' is the correct and more concise idiom ('instrumental in [doing something]'). Option C correctly uses the idiom 'was instrumental in' and is more concise than using 'an instrumental factor.' The past tense 'was' is appropriate because the historians' belief (expressed in the present perfect 'have long believed') refers to an event (the invention of the printing press) and its historical consequences (Renaissance, Reformation) that are completed. Option B uses the past perfect 'had been,' which is incorrect as there is no later past event that necessitates this tense. Option E uses the present perfect 'has been,' which implies the invention's instrumental role is still actively unfolding from the perspective of the historians' belief, which is not the case for these historical events. Options A and D retain the awkward and wordy 'an instrumental factor for.'