It is plural in this case but it can be singular too. Consider this:
The 32 species make up this dolphin family, which is finally migrating to North Sea..

Anant, A. so embarrassed in his later years by what he considered the excessive sentiment in the poems in his first two collections, he destroyed There is no verb in the original sentence. Ignore the modifier "who won the Nobel prize..." Spanish poet Juan Ramon Jimenez.....so embarrassed i...

Matt/Mitch,

Can you please throw some light on my question above?

Yes, they are definitely worth the time.

Isn't usage of immigration laws instead of immigration law in D also wrong?

I picked originally C as the answer.

In what case D be a preferred choice over C?

Experts please clarify.

Thanks in advance

Sandipgumtya,

A is wrong because latter part of the sentence uses "is" and the underlined portion "their"
Mismatch in number.

Rich,

I would post my full-test length test score shortly, had to undergo a surgery last week. Trying to recover from it. My target score is >730 and I am planning to take test on 12/01/2015.

Rich, I appreciate your response but I am not assuming anything that question does not state. My confusion is that question is ambiguous. Where in the question is clarified that socks are matched within and between the pairs? In your response you are assuming that 7 socks are removed 11, 22, 33 and ...