Zero Float:
Zero float refers to the situation where the total float of an activity is equal to zero. Total float is the amount of time that an activity can be delayed without delaying the project's overall completion date. It is calculated by subtracting the early start date of an activity from its late start date or by subtracting the early finish date from the late finish date.

Understanding Float:
Float is a concept used in project management to determine the flexibility or slack time of activities within a project schedule. It helps in identifying the critical path, which is the sequence of activities that determine the project's shortest duration. The critical path is crucial because any delay in these activities will directly impact the project's overall duration.

Types of Float:
There are three types of float in project management:

1. Total Float: It is the amount of time that an activity can be delayed without delaying the project's overall completion date. Activities with zero float are on the critical path, while activities with positive float have some flexibility in their scheduling.

2. Free Float: It is the amount of time that an activity can be delayed without delaying the early start of its successor activities. Free float is typically used to identify potential delays within a specific sequence of activities.

3. Interfering Float: It is the amount of time that an activity's start can be delayed without delaying the late start date of any successor activity. It is used to identify delays that may affect the overall project duration indirectly.

Zero Float and Critical Activity:
In a project schedule, the critical path consists of activities with zero float. These activities are critical because any delay in their completion will directly impact the project's overall duration. Therefore, it is important to closely monitor and manage the critical activities to ensure timely completion of the project.

Conclusion:
Option 'C' correctly states that the critical activity has zero float. This means that any delay in the critical activity will directly delay the project's overall completion date. Zero float activities require careful attention and management to ensure the project stays on track and meets its deadline.