He was in a brown study and did not seem to catch my point.

The idiom "in a brown study" means to be deeply absorbed in thought, often to the point of being unaware of one's surroundings or inattentive. Hence, "absent-minded" (Option C) best captures the meaning of the phrase.

Option B ("absorbed in reading") seems like a close contender, but the idiom "in a brown study" typically refers to being preoccupied with one's own thoughts rather than just focusing on 'reading.'