The answer is Choice A: (cos(theta), sin(theta))
More specifically x = cos(theta) and y = sin(theta) make up the point (x,y) which is where point P is located. Because we have a unit circle, the equation of the circle is x^2+y^2 = 1. This leads to cos^2(theta) + sin^2(theta) = 1 which is the pythagorean trig identity.