The process in which a single yarn is used to make a piece of fabric is called _______.

Knitting is the process of creating a fabric item out of a single yarn. Sweaters, cardigans, and socks are items made up of knitted fibres.

Weaving is a method of textile production in which two or more distinct sets of yarns or threads are interlaced to form a fabric or cloth.

Spinning is the process of drawing out fibers from a mass and twisting them together to form a continuous thread or yarn. 

The process of separation of cotton fibres from their seeds is called ginning of cotton.

So, the correct answer is -'Knitting'.