Question

In an open pair, the two elements of a pair

(a) have a surface contact when in motion

(b) have a line or point contact when in motion

(c) are kept in contact by the action of external forces, when in motion

(d) are not held mechanically

This question was addressed to me in a dream while sleeping

The origin of the question is Kinematic Pair in chapter Simple Mechanisms of Machine Kinematics

Answer 1

Right answer is (d) are not held mechanically

The best I can explain: When the two elements of a pair are not held mechanically, they are called open pair.