Answer: D

Reasoning: The mandibular second premolar occurs in three common occlusal configurations.  
• A three-cusp variety has a Y-shaped central groove.  
• A two-cusp variety may form either a U- (crescent) or an H-shaped central groove.  
Because any of these three groove shapes can be found, all the options listed are correct. [Wheeler’s Dental Anatomy, Physiology and Occlusion, 10th ed.; Nelson & Ash, 2019]