Final answer:
The presence of WPS support is the feature that notably weakens WPA security due to a vulnerability that allows attackers to brute-force the WPS PIN, simplifying unauthorized access to a Wi-Fi network.
Step-by-step explanation:
The feature that makes WPA easy to defeat is B). WPS support. Wi-Fi Protected Setup (WPS) is intended to simplify the process of connecting devices to a Wi-Fi network but has a well-known vulnerability. The WPS PIN, an 8-digit number used to add new devices to a network, can be brute-forced. The attack is feasible because the PIN's last digit is a checksum, and the PIN is processed in two parts, which reduces the number of attempts needed to crack it significantly.
While PSK, TKIP, and RC4 have their own individual vulnerabilities, none of these directly make WPA easy to defeat as much as the flawed implementation of WPS does.