Hi Engin,

This is a relative pronoun case error question.

The modifier " many who take expensive brand-name medications daily" is modifying elderly patients, the subject of the sentence. The underlined portion is describing many of ? . We need something which RELATES the modifier to subject so relative pronoun whom should be used. Why not WHO? Well WHO is used as a subject and we already have one subject so we need a relative pronoun on which action is performed , So whom is used.

a.many who- case error
b.many of them who - redundant and also changes the meaning of sentence
c.and many of them who- and introduces another main clause
d.many of whom - correct case
e.many of which - which generally for non-living things

Hope it helps

Thanks
Komal

Another way to think about it is to change the second part and try to use "they" or "them".

Would we say:

"many they take medicine"

or

"many of them take medicine"?

Since the second option is proper, we need to use "of whom" instead of "who".