The sentence is
According to a recent survey, the traditional language of the Saanich, a First Nations indigenous community with roots on Canada's Vancouver Island, has fewer than twenty fluent speakers. All of them over the age of sixty.
To replace "speakers. All of them"
A)NO CHANGE
B) speakers, all of whom
C)speakers; all of whom
D) speakers, all of them
I know why option A and option C are incorrect. My problem is with options B and D.
The correct answer is option D but I don't understand why option B is ungrammatical or even less favorable. If my understanding is correct, because the second clause has no verb it is a dependent clause. If it did however have a verb only option B would be correct because it would be the only dependent clause because of the relative pronoun whom. Why does omitting the verb make it not work?
Best Answer
If you have a relative pronoun, it forms the head of a relative clause. A relative clause requires a subject and a verb with tense. In our case, "all of whom" would form the subject, but there's no verb with tense to go with it, so the grammar is bad.
With "all of them", there is no such requirement for a clause. The structure [ "all of them" + adjective phrase ] is good grammar. You could also understand the structure as [ "all of them" (+ "being") + adjective phrase ], where the "being" is elided in your example.
Either way, the structure in B requires a clause, while the structure in D does not.