The number of screw threads per inch for a Standard National Coarse 1/2 inch bolt is __________.

Correct Answer: Option B — 13

Unified National Coarse (UNC) thread standards dictate the pitch for standard fasteners. For a 1/2-inch diameter bolt, the National Coarse designation specifies 13 threads per inch. This standard is critical for ensuring proper thread engagement and holding strength in marine machinery applications governed by mechanical engineering standards.