Final answer:
The Vertical Jump Test is used to determine an athlete's reactive strength.
Step-by-step explanation:
The assessment used to determine an athlete's reactive strength is the Vertical Jump Test.
In the Vertical Jump Test, an athlete jumps vertically and the height of the jump is measured. This test evaluates an athlete's explosive power, which is an important component of reactive strength.
Other assessments mentioned in the question, such as the 40-Yard Dash, T-Test, and Illinois Agility Test, assess different aspects of an athlete's speed and agility, but they do not specifically measure reactive strength.