Final answer:
QPP contributions are mandatory for all workers aged 18 or older in pensionable employment, including full-time, part-time, and seasonal employees.
Step-by-step explanation:
The Quebec Pension Plan (QPP) contributions must be withheld from all workers aged 18 or older in pensionable employment, which includes full-time employees, part-time employees, and seasonal employees. Therefore, the answer is 4) All of the above. This is because the QPP aims to provide a measure of financial protection to workers and their families against the loss of income due to retirement, disability, or death. Contributions to such a social security system are typically mandatory for all workers who meet the age and employment criteria.