Which of the following IEEE standards defines Wired Equivalent Privacy encryption scheme?

IEEE 802.15 defines Wireless Personal Area Network standards such as Bluetooth and Zigbee and has no relation to WEP or Wi-Fi security mechanisms.

WEP was introduced in the original IEEE 802.11-1997 specification and was formally carried forward and widely implemented through the 802.11b amendment, which defined the first broadly adopted 2.4 GHz Wi-Fi standard at up to 11 Mbps. Because 802.11b was the standard under which WEP became ubiquitous in commercial products, it is the standard most commonly cited in certification contexts when referencing WEP's definition and deployment.

IEEE 802.11a defines a 5 GHz OFDM-based Wi-Fi standard that does not independently define WEP, which originated in and is associated with the 2.4 GHz 802.11 lineage.

IEEE 802.11g is a later 2.4 GHz standard that improved data throughput over 802.11b but inherits WEP from earlier specifications rather than defining it.