The correct answer is B. with.
The sentence “He went so fast that I could not keep up with him” means that the speaker was trying to follow the person who was going fast, but they were not able to do so. The word “with” indicates that the speaker was trying to stay close to the person who was going fast, but they were not able to do so.
The other options are incorrect because they do not convey the same meaning as the sentence. The word “for” indicates that the speaker was trying to do something for the person who was going fast, but they were not able to do so. The word “to” indicates that the speaker was trying to go to a place where the person who was going fast was going, but they were not able to do so. The word “in” indicates that the speaker was trying to be in a place where the person who was going fast was, but they were not able to do so.