A pair of equal and opposite non-collinear parallel forces forms a couple.

Let the two forces be $F$ and $-F$, acting along parallel but different lines of action.

Step 1: Consider the net force

Since the forces are equal in magnitude and opposite in direction,

$$F_{\text{net}} = F + (-F) = 0$$

So there is no resultant force on the body.

Because the net force is zero, the body will not undergo translational motion due to these forces.

Step 2: Consider the turning effect

Although the net force is zero, the forces act along different lines of action. Since they are non-collinear, they produce a turning effect.

The moment of a couple is given by:

$$\tau = Fd$$

where:

• $\tau$ is the torque or moment of the couple,
• $F$ is the magnitude of one of the forces,
• $d$ is the perpendicular distance between the two lines of action.

Since $F \neq 0$ and $d \neq 0$,

$$\tau \neq 0$$

So the forces produce a non-zero torque.

Step 3: Determine the type of motion

A non-zero torque causes a body to rotate. Therefore, the pair of equal and opposite non-collinear parallel forces produces rotational motion.

Hence, the correct answer is:

(D) Rotational motion