Areaction was found to be second order in carbon monoxide concentration. the rate of the reaction if the [co] is doubled, with everything else kept the same.

The rate of the reaction increases by a factor of 4. because the reaction found to be second order in CO so, we take [CO]^2 and when the concentration is doubled so [ 2]^2 = 4 that's why ( increases by a factor of 4) is the true answer.