Free float is the

Free float is a concept in Critical Path Method (CPM) scheduling that refers to the amount of time an activity's early start can be delayed without affecting the early start of any immediately succeeding activities.

Option A refers to total float, which considers early and late dates of an activity​.

Option C also points to total float.

Option D does not define free float.

Free float is critical for project planners as it provides flexibility in scheduling without impacting downstream activities​.