In an open pair, the two elements of a pair

>In an open pair:

Option A: Have a surface contact when in motion. This is incorrect because surface contact typically describes a closed pair.

Option B: Have a line or point contact when in motion. This is incorrect as it typically describes higher pairs, not open pairs.

Option C: Are kept in contact by the action of external forces, when in motion. This is correct because, in an open pair, external forces are necessary to maintain contact between the elements.

Option D: Are not held together mechanically. This is incorrect because while they are not mechanically locked, they are held together by external forces.

Therefore, the correct answer is Option C: Are kept in contact by the action of external forces, when in motion.