The correct answer is A. Channel overlap. The signal strength numbers given, -50 to -65 dBm, are generally strong enough for normal client connectivity, so this does not point to weak coverage. The bigger clue is that the environment is a multitenant office building and the wireless network is using 2.4 GHz. That band is well known for congestion and overlapping channels, especially where many nearby businesses or tenants have their own access points.

In the 2.4 GHz range, there are only a few non-overlapping channels commonly used. When many APs operate close together on overlapping channels, clients can experience interference, retransmissions, poor throughput, buffering, and connection instability. Those symptoms fit the scenario very well.

The other options are less convincing. Insufficient wireless coverage does not line up with the reported signal levels. AP antenna type could matter in some designs, but there is nothing in the question pointing to a directional or placement issue. WPA2 is not the cause of the performance symptoms here; while it is older than WPA3, it does not normally create repeated buffering and disconnections in this way.

This is really a wireless interference question, and in a crowded 2.4 GHz building, channel overlap is the most likely reason.