Final answer:
The correct option is A: Channel Bonding. Channel bonding is the 802.11n feature that allows the combination of two 20-MHz channels to create a 40-MHz channel, increasing network speed.
Step-by-step explanation:
The feature in 802.11n that allows two adjacent 20-MHz channels to be combined to make a 40-MHz channel is known as Channel bonding. This technique effectively doubles the channel width, which can increase the network throughput and provide a faster data transmission rate. Channel bonding is particularly useful in environments where higher wireless speeds are needed and the additional spectrum is available.