GRE problem 55: why is interference not the answer?

[Silviu]

Hello! I have a question about a GRE problem. It is problem 55 in the attached file. So the answer seems to be (A) but I am a bit confused. Isn't this basically interference, so the right answer would be (C)?

 

[blue_leaf77]

Remember that the two EM fields are perpendicular to each other. Since after the mentioned OPD the waves are in phase, the resultant field at that point in space will be $\mathbf E(t) = \hat x E_1 \cos(\omega t + \phi) + \hat y E_2 \cos(\omega t + \phi)$ where $\omega$ and $\phi$ are constants. What is the length of this vector?