A trial period of work is referred to as O probationary period
trial period
prework
o on-the-job-training period

O probationary period

Explanation:

Probation refers to the set amount of time that an employee needs work before they are made permanent. The objective of the probation period is to give the employee time to learn the business process and procedure.  Probation enables employers to evaluate the new employees' work rate, discipline, habits, and attitudes before deciding whether to hire them as permanent workers or not.

From the data collected at probation, the employer may decide to promote the employee to permanent status, extend the probation period, or discontinue the worker. An employee under probation enjoys a salary and other benefits as the employer may decide.