Who was observed unloading supplies at the school?

The correct choice highlights Harry, the teaching assistant, as the individual observed unloading supplies at the school. This choice underscores the roles that various staff members have within a school setting. Teaching assistants often engage in activities that support the educational environment, including logistical tasks such as unloading supplies.

In a school, the role of a custodian typically involves maintenance and cleaning rather than direct involvement in supply management. The school principal generally has administrative responsibilities and is less likely to be involved in manually unloading supplies. The mention of another student also does not align with the likely responsibilities, as unloading supplies is typically not a task assigned to students. This contextual understanding reinforces why Harry, the teaching assistant, would be the most plausible individual noted for unloading supplies, as his role would often encompass such support tasks.