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.