In a simple Harmonic motion, Force must be linearly proportional to the displacement.

Also, F = -kx

a = -ω²x

(a).

Direction of Force and acceleration is always same. So, the

dot product will become positive as for <90° ,is positive.

So this expression is positive.

(b).

Direction of velocity and displacement is same so the expression will also be positive.

(c).

Force is opposite to displacement.

Dot product will become negative as for >90° ,is negative

So, this expression is negative

Again,

(d)

Force is opposite to displacement so it will also be negative.

Therefore, the option a and b are correct.