In option (a), the state at t=0 and t=2 are not the same as the displacement are not equal. Thus incorrect.

In option (b), the state at t=2s and t=6s are in the same state as the displacement and direction are the same. Thus correct.

In option (c), though the displacement is same at t=1s and t=7s but the direction is opposite. Thus incorrect.

In option (d), the state at t=1s and t=5s are in the same state as the displacement and direction are the same. Thus correct.

∴ the option (b) and (d) are the correct.