The correct answer is A. Straight line.
The instantaneous center of rotation (ICR) of a rigid body is the point about which the body appears to rotate at a given instant. The ICR is always located on the line of action of the net external force acting on the body.
The locus of the ICR is a straight line called the instantaneous axis of rotation. The instantaneous axis of rotation is always perpendicular to the plane of motion of the body.
The involute of a curve is the locus of all points on a curve that are tangent to a given curve. The centroid of a plane figure is the point of intersection of its medians. A spiral is a curve that winds around a central point, getting closer and closer to it as it goes.
In the case of a moving rigid body, the ICR is always located on the line of action of the net external force acting on the body. This line of action is always perpendicular to the plane of motion of the body. Therefore, the locus of the ICR is a straight line.