The most common shape of nut is

Square

Hexagonal

Round

Flat

Answer is:

Hexagonal

Explanation:

The most common shape of nut is Hexagonal

It is semi-permanent fastener

It is headless

It is generally used to screw one object within another

It is unable to prevent relative motion between two rotating parts

Answer is:

Explanation:

grub screw

set screw

stud

thumb screw

Answer is:

Explanation:

A small headless screw having a slot cut for a screw driver is called grub screw

Hexagonal

Square

Circular

Triangular

Answer is:

Explanation:

The head of a thumb screw is circular in shape.

Gib head key

Feather key

Woodruff key

Saddle key

Answer is:

Explanation:

A key that fits into a semicircular keyway cut in the shaft is woodruff key