SAT grammar question: Why is this "them" incorrect?

It should be most of whom, not most of *them.

Otherwise it is a comma-splice error caused by incorrectly attempting to join together two independent clauses with a mere comma and no conjunction.

These are all correctly formed:

1. There are more than 300 million English speakers in India. Most of them acquired English as a second language.

2. There are more than 300 million English speakers in India; most of them acquired English as a second language.

3. There are more than 300 million English speakers in India, but most of them acquired English as a second language.

4. There are more than 300 million English speakers in India, most of whom acquired English as a second language.

Without a semicolon, it cannot be them; with a semicolon, it cannot be whom.

The sentence can be recast with both "them" and the comma still utilized:

There are (more than) 300 million English speakers (in) India, most of (them) having acquired English (as) a second language.

Surely, I do not pretend this would have been a choice available in the real test, still I think it makes an useful point in terms of alternative phrasing.