In time–cost analysis for a single activity, the time corresponding to the least possible direct construction cost of that activity is termed as:

Introduction / Context:
Time–cost trade-off examines how changing an activity duration affects its direct cost. Generally, expediting an activity requires additional resources and raises direct cost, while excessively stretching time may also increase indirect or inefficiency costs. The question asks for the term used for the duration with the least direct cost for the activity itself.

Given Data / Assumptions:

• We consider a single activity's direct cost curve.
• Crashing shortens duration by adding resources and higher unit costs.
• Normal time is the planned, economical duration before any crashing.

Concept / Approach:
Direct cost typically increases when we crash (shorten) the activity. The baseline economical duration that yields the minimum direct cost is called normal time. Crash time is the minimum feasible duration but at a higher direct cost. Terms like slow time or standard time are not standard in CPM time–cost nomenclature for the least direct cost point.

Step-by-Step Solution:
Relate definitions: normal time = planned economical duration; crash time = expedited minimum duration.Identify which corresponds to least direct cost: normal time.Confirm that crashing raises direct cost even if it may lower project overheads.

Verification / Alternative check:
Typical time–cost graphs show a convex or piecewise linear rise in direct cost as duration approaches the crash limit from normal time.

Why Other Options Are Wrong:

• Slow time: Not a standard CPM term for cost optimum.
• Crash time: Minimum duration but highest direct cost.
• Standard time: Generic term not tied to cost optimum in CPM context.

Common Pitfalls:
Confusing project total cost optimum with activity direct cost minimum; assuming shortest time always saves money.

Final Answer:
Normal time