A student writes four different expressions for the displacement y in a periodic motion as a function of time t, a as amplitude, T as time period. Correct options are

Since LHS is displacement, so RHS should have dimensions of displacement. In (1), aT does not have the dimensions of displacement. Also the argument of a trigonometric function should be dimensionless. In (2), argument is not dimensionless and in (3), a/T does not have the dimensions of displacement. Hence, the correct choice is (4).