Pre-tensioning of a bolted joint is used to

The correct option is: increase stiffness of the bolted joint

Explanation:

Pre-tensioning (also known as pre-loading) of a bolted joint involves tightening the bolt to a specific initial tensile force during assembly. This creates a compressive clamping force on the connected members of the joint.
When an external tensile load is subsequently applied to the joint, it does not act entirely on the bolt. Instead, because of the initial clamping force, the external load is shared between the bolt and the clamped members.

Specifically, the overall stiffness of the joint assembly is a function of the combined stiffness of the bolt and the members. Pre-tensioning ensures that the members remain tightly clamped together under load. Since the clamped members typically have a much larger cross-sectional area and higher stiffness compared to the bolt, keeping them in compression significantly increases the overall structural stiffness of the joint and reduces the amplitude of alternating stress in the bolt, thereby improving fatigue life and preventing joint separation.