Since the start of space age, more and more littering has occurred in orbits near Earth, often because the intentional discarding of lens caps, packing material , fuel tanks and payload covers.

a) more and more littering has occurred in orbits near Earth, often because
b) orbits near Earth have become more and more littered often from
c) orbits near Earth became littered more and more, often resulting from
d) there have been more and more littering of orbits near Earth, often because of
e) there had been littering more and more of orbits near Earth, often with

because + of must be followed by a NOUN or NOUN PHRASE.
because + NO of must be followed by a COMPLETE CLAUSE.
In A, because + NO of is not followed by a complete clause.
Eliminate A.

The agent of a COMMA + VERBing modifier must be the nearest preceding SUBJECT.
In C, resulting seems to refer to orbits -- the nearest preceding subject -- implying that ORBITS are RESULTING from the intentional discarding of lens caps.
This meaning is nonsensical.
Eliminate C.

In D, have been (plural) does not agree with littering (singular).
Eliminate D.

In E, had been littering (past perfect) implies an action completed IN THE PAST, while since the start of the space age implies an action still happening IN THE PRESENT.
Since the two usages contradict each other, eliminate E.

The correct answer is B.

Hi Mitch ,

Thank you so much for your reply.

Can you please explain more about the below part. It will be really helpful.

because + of must be followed by a NOUN or NOUN PHRASE.
because + NO of must be followed by a COMPLETE CLAUSE.
In A, because + NO of is not followed by a complete clause.
Eliminate A.

Thanks..[/i]