Trigonometry , question posted below @al2006

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.