A kinematic chain is known as a mechanism when

A kinematic chain is a combination of links connected in a specific order, allowing relative motion between them. When one of the links in a kinematic chain is fixed or stationary, it is termed a mechanism. This fixed link provides a frame of reference against which the motions of other links are observed. The fixed link is often referred to as the "ground" or "frame" link, and it serves as a foundation for the movement and analysis of the remaining links in the kinematic chain.

By fixing a single link, the entire kinematic chain becomes a functional system with controlled or constrained motion. This is a fundamental concept in the study of mechanisms, which are essential components in various mechanical devices and machines used in engineering applications.

Note: Options A and C are not accurate because the presence of fixed links is essential to define a mechanism. Option D is also not correct as one of the options (Option B) is indeed the correct condition for a kinematic chain to be termed a mechanism.