Hi Mr. Fullerton,
Thank you so much for your swift response. However, I am not sure I understand what you mean in:
To choose between A or D, look at the function you're using to model the waves and its value at time t=0. With a phase change of 5Pi, you'll have the opposite situation you would have at time t=0. So for a basic cosine function modeled so that the E field has a maximum value at time t=0, at the instant in time where the phase is 5Pi, it would be at a minimum (0) value, so the answer would be A. If you are modeling the wave so that the minimum value is exhibited at time t=0, you would have a maximum magnitude at phase 5Pi.
How do you know that the model of a Electromagnetic wave is cosine? Also, how do you know that the mininum vector is at 0(I know this is the only option but is there a way you would know this if the minimum was not given to you)?
Thank you so much for taking the time to answer my question!