And Most of them or Most of whom

The correct answer is B. "Peter invited lots of guests to his party, most of them from his baseball team" - it means most of the guests Peter invited were from his baseball team. A and D don't work, because it doesn't make sense to use "and" there. I'm assuming this is a fill-in-the-blanks/cloze exercise, where you can't rephrase the sentence, so C doesn't work either. If you *could* rephrase, you could say "Peter invited lots of guests to his party, most of whom *were* from his baseball team". Same meaning as B but without "were", which is not in the text you were given, it doesn't work.

Like the other commenter said B is correct. The and isn't required and for C the words added to make it work are not present. B would be what I would say as well!

a and c are wrong - because of and. The correct answer is B.

From all the choices, I would say B would be the correct answer. The use of whom from choice C and D would not pair well here. “Whom” is an object pronoun whereas “who” is a subject pronoun. Neither of these would fit as we are talking about a group. The comma acts as a pause between the two sentence structures to differentiate between sentence. I think I made that more complicated than it needed to be, but in conclusion informally when speaking outside of a professional setting many people do not use “whom”, when speaking informally to each other. Formally in settings that are professional (legal, workplace, and more), whom can be used when referring to a person in place of pronouns like her or him. Please excuse my lack of punctuation in certain places in this response.