Explanation
The clients are 802.11n devices. 802.11n devices can only support a maximum of four spatial streams, and the maximum data rate for a single spatial stream is 150 Mbps. Therefore, if most clients are only achieving 150 Mbps, it means they are using only one spatial stream, which is typical for 802.11n devices. To achieve higher data rates, the clients would need to support more spatial streams, which is possible with 802.11ax devices12.
The other options are not correct because they do not explain why most clients are only achieving 150 Mbps. Option B is incorrect because two stream 802.11ax clients can achieve data rates of up to 574 Mbps in a
20 MHz channel1. Option C is incorrect because contention caused by an overlapping BSS would affect all clients, not just most of them, and it would also reduce the throughput, not the data rate. Option D is incorrect because non-Wi-Fi interference in the channel would also affect all clients, not just most of them, and it would also cause errors and retransmissions, not lower data rates.
References: 1: CWNA-108 Official Study Guide, page 144 2: 802.11ax data rates