Rahul and his friends . . . . . . . . also invited to the party.

Answer: Option D

In such sentences where an individual is mentioned along with a group, we check the subject-predicate relation and try to figure out which is the subject. Like here Rahul and his friends are the focus of the sentence or the subject therefore the verb used will be plural. As a number of individuals are being discussed. Hence, 'were' is the correct answer.