Though he possessed a formidable intellect, the professor's lectures were often considered _______ due to his tendency to digress and his convoluted explanations, which frequently obscured rather than clarified the topic.
✓ Correct Answer: Option D
The professor's explanations were "convoluted" and "obscured rather than clarified." This means his lectures were difficult to understand or grasp, making them "recondite" (abstruse or profound).