The answer is C. But let me explain to you why.
For letter A, this is not true. If your are talking about the angle of depression of the plane, that would be ∠1.
For letter B, this is not true because angle of elevation would indicate the the tower is higher than the airplane, which is obviously impossible.
For letter D, this is the opposite of letter B. Angle of depression would mean that the airplane is below the tower.
I hope my explanation helped you in your understanding of this problem :)