Which of the following quantities are always negative in a simple harmonic motion?

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

F =-kx

(a). = Cosθ

Direction of Force and acceleration is always same. Therefore, it will always be positive.

(b).

Direction of Velocity and displacement is always same; thus it is also coming 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, Option (c). and (d) are correct.