Answer: 3- The average speed of the molecules would reduce.
It has been mentioned that the container in which the ideal gas is cooled from 50 degree Celsius to 25 degree Celsius has constant volume. Since the volume of the container is constant, the molecular mass would remain the same. Also since the volume is constant even though the gas is cooled down,the average distance between the molecules would remain the same.
Since the temperature is reduced from 50 degree Celsius to 25 degree Celsius, the movement and the kinetic energy associated with the gas molecules would reduce. Hence the average speed of the molecules would reduce.