The Beatles were to be honored on account they bolstered the sagging B

Official Explanation

The underlined portion of this sentence is a subordinate (or dependent) clause, that is, one that cannot stand by itself as a complete sentence but which must be joined to an independent or other dependent clause by a subordinate conjunction.

Both (C) and (E) introduce the clause by a subordinate conjunction (when and since). Since the relationship between the two clauses of the sentence is one of cause and effect, since is a better word than when. On account, if used at all, should be in the form on account of; furthermore, since of is a preposition, it must take an object, for example, “their bolstering of the sagging British economy.” Being that is not acceptable in Standard Written English as a substitute for since or because.

Choice (D) is redundant (the reason being and on account of say the same thing).

The correct answer is (E).