It's ' D '.
-- Period: The angle is (pi/2 · t).
From t=0 to t=4 sec, the angle changes by 2pi. So the period is 4 sec.
-- Amplitude: The biggest the cosine can ever be is 1 .
When the cosine part is 1, 'd' is 6, so the amplitude of the model is 6 .
-- Position when t=0:
When t=0, the angle in the cosine is zero.
The cosine of zero is 1 .
So when t=0, d=6 . There you are.
