The correct answer is: 1.5 mm.
The gross diameter of a rivet is the diameter of the unheaded portion of the rivet. The nominal diameter is the diameter of the hole that the rivet is driven into. The difference between the gross diameter and the nominal diameter is called the rivet allowance. The rivet allowance is typically 1.5 mm for rivets up to 25 mm diameter.
The rivet allowance is necessary to ensure that the rivet is properly seated in the hole and that there is enough material around the rivet to prevent it from pulling through the material. The rivet allowance is also necessary to provide a space for the rivet to expand when it is driven.
The rivet allowance is typically specified in the rivet manufacturer’s specifications. The rivet allowance should be taken into account when designing a riveted joint.