B because in the sentence, generous is used as an adjective to the boy and also the word new and red describes the ballpen which is right. as you can see the letter a it states that he lent his generous classmate. is that statement right? the answer is no and on the other hand letter c, if you're going to construct that sentence, generous is not used as an adjective but an adverb which makes it generously. so letter c is not also right then you can conclude that b is the right answer.