C: not only are thieves...also pilfering
D: not only are thieves...but also pilfer
E: not only are thieve...but also pilfering
In these options, the portions in red do not have both a subject and verb and thus do not constitute clauses.
Eliminate C, D and E.

fiza gupta wrote:but in option D: not only are thieves...but also pilfer
isn't the word "pilfer" a verb?
if i google, it says its a verb

In D, not only + VERB + SUBJECT is followed by a verb (pilfer) without a subject.
As noted in my post above, not only + VERB + SUBJECT must be followed by a clause with its own subject and verb.
The subject in the second clause may NOT be omitted.
Since the second clause in D lacks a subject, eliminate D.

but :
this is one of the question in OG 2017 in which subject is missing but still the construction is correct
"Whether they will scale back their orders to pre-2003 levels or stop doing business with us altogether depends on whether the changes that their management has proposed will be fully implemented."
why this construction is acceptable without subject?

richachampion wrote:As criminal activity on the Internet becomes more and more sophisticated, not only are thieves able to divert cash from company bank accounts, they can also pilfer valuable information such as business development strategies, new product specifications, and contract bidding plans, and sell the data to competitors.

A. they can also pilfer valuable information such as business development strategies, new product specifications, and contract bidding plans, and sell
B. they can also pilfer valuable information that includes business development strategies, new product specifications, and contract bidding plans, and selling
C. also pilfering valuable information including business development strategies, new product specifications, and contract bidding plans, selling
D. but also pilfer valuable information such as business development strategies, new product specifications, and contract bidding plans to sell
E. but also pilfering valuable information such as business development strategies, new product specifications, and contract bidding plans and selling