Number is the subject and doubled is the verb. 'Was' is unnecessary.

Mechmeera wrote:The number of undergraduate degrees in engineering awarded by colleges and universities in the United States increased by more than twice from 1978 to 1985.

a.increased by more than twice
b.increased more than two times
c.more than doubled
d.was more than doubled
e.had more than doubled

OA is C Can you explain why D is wrong

The word 'doubled' with the auxiliary 'was' is a verb of incomplete predication, which means that we need a complement to complete the meaning of the verb phrase. The verb phrase suggests that someone or something else doubled the number. So we need to provide the complement to remove the ambiguity. But, without the auxiliary 'was' there is less scope for ambiguity with the word 'doubled', as the sentence now suggests that: the number just doubled without any need for further explanation.

Also, the construction 'be + past participle' conveys a passive structure. So, the construction 'was doubled' is a passive construction. The GMAT, generally, avoids the passive construction and prefer the active one. However, you can't dismiss an option solely based on active/passive criteria; you must evaluate all other possible grammatical reasons to reject the option. Option C with the simple past is in active voice and also meets the grammatical criteria,I mentioned in the previous post, to be the better option.