Approximately fifty percent of the calories in this candy bar is from

Official Answer:

Sentence Correction questions with subjects that represent portions of something (such as percent, portion etc...) can be singular or plural. You determine whether it is singular or plural depending on the object of the preposition which follows. For instance fifty percent of the pie is singular while fifity percent of the pies is plural. This is an often used trick on the GMAT and one to watch out for.

In this question there are 50 percent of the calories and since calories is plural then the subject is plural and needs the plural "are" in the answer. This eliminates A, B and C as possible answers.

The next issue is whether the verb "provides" should be plural or singular. The "which" that begins the clause changes the subject of the clause to peanuts, as opposed to percent. Thus the verb needs to be plural. the plural form is the verb "provide" and thus D is the best answer.