The correct answer is A. Start float and finish float are always equal.
Start float is the amount of time that an activity can be delayed without delaying the start of any succeeding activities. Finish float is the amount of time that an activity can be delayed without delaying the finish of any succeeding activities.
Start float and finish float are not always equal. For example, consider the following network diagram:
Activity A has a start float of 2 days and a finish float of 1 day. Activity B has a start float of 1 day and a finish float of 0 days. Activity C has a start float of 0 days and a finish float of 2 days.
As you can see, start float and finish float are not always equal. In this example, activity A has a start float of 2 days, but a finish float of only 1 day. This means that activity A can be delayed by up to 2 days without delaying the start of any succeeding activities, but it can only be delayed by up to 1 day without delaying the finish of any succeeding activities.
B. Total float can be either start float or finish float. This is true. Total float is the amount of time that an activity can be delayed without delaying the project completion date. It is equal to the sum of the start float and the finish float.
C. Start float and finish float need not be equal. This is also true. As explained above, start float and finish float are not always equal.
D. Start float and finish float are the differences between activity times and not event times. This is also true. Start float and finish float are the amounts of time that an activity can be delayed without delaying the start or finish of any succeeding activities, respectively. They are not the differences between activity times and event times.