ANS:A - grain size.

Nitriding of a steel part does not increase its grain size. Nitriding is a surface-hardening process that involves the diffusion of nitrogen into the surface of steel parts. This process forms hard nitride compounds on the surface, which enhances properties such as surface hardness, wear resistance, and fatigue limit. However, it does not significantly affect the grain size of the steel. Grain size is primarily determined during the steel manufacturing process and is not altered by nitriding. Therefore, the correct option is: grain size.