Which of the following standards can operate at either 20MHz or 40MHz channel spacing?

A.
802.11a

B.
802.11b

C.
802.11g

D.
802.11n

Explanation:

The 2.4 GHz ISM band is fairly congested. With 802.11n, there is the option to double the
bandwidth per channel to 40 MHz which results in slightly more than double the data rate.
However, when in 2.4 GHz, enabling this option takes up to 82%[10] of the unlicensed band,
which in many areas may prove to be infeasible.
The specification calls for requiring one primary 20 MHz channel as well as a secondary adjacent
channel spaced ±20 MHz away. The primary channel is used for communications with clients
incapable of 40 MHz mode. When in 40 MHz mode, the center frequency is actually the mean of
the primary and secondary channels.