21. the number of subordinates that one supervisor can manage effectively during an incident is referred to as:
(a) unity of command
(b) management by objectives
(c) manageable span of control
(d) modular organization

(C) Manageable Span of Control

Explanation:

According to my research on managerial terminology, I can say that based on the information provided within the question this description is referring to the term known as Manageable Span of Control. The amount of people they  can manage may vary depending on the type of incident, nature of the task, hazards and safety factors, and distances between personnel and resources.

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(C) Manageable Span of Control

Explanation:

The manageable span of control refers the amount of subordinates that a supervisor can manage effectively in a given circumstance, and it is often 5 subordinates for every supervisor. So the span would be 5:1 and it is often adviced for any type of institution and organization.