The professor's lecture on quantum physics was so ____________ that only a handful of advanced students could grasp its complexities.
Correct: C
The context indicates that the lecture was extremely complex and difficult to understand, even for advanced students. 'Recondite' means difficult or impossible for one of ordinary understanding or knowledge to comprehend; esoteric. This fits the description. 'Lucid' means expressed clearly; easy to understand, which is the opposite. 'Pedestrian' means lacking inspiration or excitement; dull. 'Ubiquitous' means present, appearing, or found everywhere.