Final answer:
The term that refers to the use of workers in the gig economy is freelance labor.
Step-by-step explanation:
The term that refers to the use of workers in the gig economy is a) Freelance labor.
Gig economy refers to a labor market characterized by temporary, flexible jobs rather than permanent employment. Freelance labor is a specific term used to describe workers who are self-employed and offer their services on a temporary or project basis. Other terms mentioned in the options, such as contingent workforce, temporary employees, and on-call workers, may also be used to describe certain types of workers in the gig economy, but the most directly relevant term is freelance labor.