I racked my brains to solve this difficult problem.

The phrase "I racked my brains" means I subjected my mind to hard thinking. It describes intense mental effort or strain to figure something out.

Option A: This implies research or study, not the mental struggle suggested by "racked my brains."

Option B:  This involves seeking external help, while "racked my brains" focuses on personal effort.

Option C: This suggests a simpler, intuitive approach, not the intense thinking implied by the phrase.
Therefore, Option D is the correct answer.