A 62 year old male comes to the GP complaining of double vision while climbing downstairs. Which of the following nerve is most likely involved?

The key is B. Trochlear nerve. [oculomotor may cause palsy of inferior rectus, medial rectus and superior rectus causing double vision in multiple gaze! But trochlear involving superior oblique only causes diplopia in downgaze only. So the answer is Trochlear nerve].