Anumāna, or inference, is a significant aspect of Indian philosophy, particularly in the Nyaya school of thought. The example provided demonstrates the classical structure of anumāna, which typically consists of three parts: the proposition (pratijñā), the reason (hetu), and the universal concomitance (vyāpti).

1. Statement (a): The mountain is fiery (proposition or pratijñā).
2. Statement (b): Because it has smoke (reason or hetu).
3. Statement (c): Wherever there is smoke, there is fire (vyāpti or universal concomitance).

Here, the focus is on finding out which option correctly represents the vyāpti, which is the universal relation expressed in the form of "wherever there is smoke, there is fire." This fundamental relation shows a necessary connection that holds true across observed instances, forming the basis of a reliable inference.

Let's analyze the options:

1. The relation of invariable concomitance between mountain and fire: This option relates the mountain and fire directly, which is incorrect because vyāpti should relate smoke and fire, not a specific instance like a mountain.
2. The relation of invariable concomitance between mountain and smoke: This suggests a connection between mountain and smoke, which doesn't represent vyāpti. Vyāpti must be a universal connection, not contextual or location-specific.
3. The relation of invariable concomitance between fire and smoke: This is correct because it reflects the universal rule that wherever there is smoke, there is fire. This is the essence of the vyāpti.
4. The relation of invariable concomitance between perceiver and the smoke: This relates an observer and smoke, which is entirely irrelevant to vyāpti, as vyāpti should depict the connection between the observed phenomenon (smoke) and its consequence (fire).

Therefore, the correct expression of vyāpti is the relation of invariable concomitance between fire and smoke.