This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'Which is the commonest diameter of engagement: \nSelect and explain. \nA. Suboccipitofrontal, B. Mentoveical, C. Occipito frontal, D. Submento veical', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Suboccipitofrontal', 'role': 'assistant'}]