The correct answers are c and d.

Although the upper portions of a tall thunderstorm next to the radar would be in the cone of silence, the lower portions of the storm would be observable by low angle scans. A tall thunderstorm that was 50 km (31 mi) away from the radar would be well observed. A low, thin cloud far away from the radar would probably be under even the lowest 0.5° scan, and would go unnoticed. Beam blocking from a nearby mountain would probably obstruct views of rain on the other side.