Answer: D. Lower the temperature of gas B.
Explanation: The average speed of the gas is directly proportional to the temperature of the gas. Increasing the temperature increases the average speed and decreasing the temperature decreases the average speed.

R= gas constant
T= temperature of the gas
M= molecular mass
The average speed of gas A is 700 m/s and gas B has a average speed of 1100 m/s. Thus to match the average speed of both, the temperature of gas B is lowered such that the average speed also reduces.