Which of the following wireless standards operates either exclusively or optionally in the 5.4GHz spectrum?

802.11b operates exclusively in the 2.4 GHz band and has no 5 GHz capability, so pairing it with 'a' is incorrect.

802.11g operates only in the 2.4 GHz band and does not support 5 GHz operation.

802.11a was designed solely for the 5 GHz spectrum, avoiding 2.4 GHz congestion. 802.11n is a dual-band standard that can operate in both 2.4 GHz and 5 GHz, making it the only other listed standard with an optional 5 GHz mode.

802.11g does not support 5 GHz, so a pairing of 'g and n' is incorrect because only 'n' of that pair uses 5 GHz.