Only C uses the plural word are referring to transfers. Therefore, C is the right answer.
In D, are seems to refer to years, therefore this is incorrect.
The total remittance transfers sent across the world from the top three industrial nations in recent years is almost $100 billion annually, according to the Government Budget Office, or roughly the equivalent of those three countries’ foreign assistance budget.
(A) years is almost $100 billion annually, according to the Government Budget Office, or roughly the equivalent of those three countries’ foreign assistance budgets.
(B) years is almost $100 billion annually, according to the Government Budget Office, roughly the equivalent of those three countries’ foreign assistance budgets.
(C) years are almost $100 billion annually, according to the Government Budget Office, or roughly the equivalent of those three countries’ foreign assistance budgets.(D) years – almost $100 billion annually – are, according to the Government Budget Office, roughly the equivalent of those three countries’ foreign assistance budgets.
(E) years, an amount that is almost $100 billion annually, are, according to the Government Budge Office, roughly the equivalent of the foreign assistance budgets of those three countries.