The argument of a trigonometric function,i.e., angle is dimensionless.
(a) [2πt/T] = [M0L0T0]
∴ |y| = |a|. So it is correct.
(b) vt is not dimensionless. So, it is wrong.
(c) t/a is not dimensionless,So,it is wrong.
(d) 2πt/T is dimensionless, So,it is correct