A spring block system is doing SHM with Amplitude A and angular frequency ω.
At t=T, the particle is at extreme position. Write displacement versus time equation.
Generally if particle A at mean position its phase constant is 0 and hence we write x = A sin ωt
But if the particle is at extreme position then the phase constant will be Π2 as you can see in the circle diagram.
So equation becomes x=Asin(ω t+Π2)
Or x=Acosω t
Hence both (c) and (d) are correct.