In project management, we calculate slack $($en$.$ slack time$)$ to identify flexibility in the planning of project elements. But what does it mean when the calculated slack is zero?

This task can be postponed indefinitely without affecting the completion of the project.

The project has the highest priority and must be completed before all other projects.

The task is optional and can be completed if necessary to pass the project completion.

The project is on the critical path and any delays that occur will delay the completion of the project.