What is Critical Path?
The critical path is the longest sequence of dependent activities through a project — and therefore the shortest possible project duration. Every activity on it has zero float: delay any one of them and the finish date moves, day for day.
Exam instinct: when a scenario asks where to focus schedule attention, apply resources, or assess a delay's impact, the answer runs through the critical path. A delay on a non-critical task with plenty of float may need no action at all.
Worked example
Tasks A(3d) → B(5d) → D(4d) form one path (12 days); A(3d) → C(2d) → D(4d) forms another (9 days). The critical path is A–B–D: 12 days. Delay B by one day and the whole project slips one day — B has zero float.