This
question asks about OAT changes at constant CAS again. You can discount two of
the options because, as above, you know that MN is constant. So you want the
relationship between CAS and TAS. The best way to think about this is the
formula Dynamic pressure = 1/2 x density x TAS^2 but I think of constant CAS
as constant dynamic pressure. So, if air becomes less dense, the only other
thing that can change in this equation is the TAS increasing to compensate.