To determine what happens when a cap screw is tightened, we need to understand the mechanics of what a cap screw is and how it functions in mechanical assemblies.

A cap screw is typically used to hold two or more objects together by applying pressure. The tightening of the cap screw causes the head of the screw to push down against one of the surfaces being joined, producing compressive force, and drawing the components together. This mechanical operation is known as a "clamping action." Let’s explore the options provided in the question:

1. Sliding motion: This type of motion isn't produced by tightening a screw. It usually involves surfaces moving over each other, which is not the intended function of a screw.
2. Clamping action (Correct Answer): Tightening a cap screw pulls the joined parts together, maintaining structural integrity and alignment through compressive force. This is explicitly known as clamping, which is the primary purpose of using a screw in fastening.
3. Cutting action: Cutting involves removing material, which a screw does not perform during tightening. Instead, screws are meant to hold material in place.
4. Rotational motion: While the screw is rotated to tighten it, the resultant action is clamping, not mere rotation. The rotation facilitates the clamping action but isn't the produced effect.

Therefore, the correct answer is: Clamping action.

Conclusion:

When a cap screw is tightened, it results in a clamping action. This is achieved because the screw draws the components together through the application of a compressive load, thereby securing them in place.