Answer
2
Work Step by Step
It can't be mechanism 1 because the rate for the elementary step would be $r=k[CO][NO_2]$
Mechanism 3 would have the rate be $r=k[NO_2]$, which is the rate for the elementary slow step.
Mechanism 2 has the rate of the slow, rate-determining step, equal to $r=k[NO_2]^2$, so this is the one that best agrees with the data.
