Answer
(a) dark cloudy day
(b) I
Work Step by Step
(a) We know that $\theta_{min}=\frac{1.22\lambda}{D}$. This equation shows that the wider the pupil diameter, the smaller will be $\theta_{min}$ (higher resolution). Since the diameter of the pupil will be opened wider in the dark, hence the resolution of the eye will be increased and the eyes will have a greater resolution on a dark cloudy day.
(b) We know that the best explanation is option (I) -- that is, your eyes have greater resolution on a cloudy day because your pupils are open wider to allow more light to enter the eye.
