Force is a term that
a unchanging over time - like balancing
b movement around a center point - like throwing
c strength used in an action - like lifting weights
d speed over time - like running

Option d , speed over time - like running

Explanation:

Force is mathematical terms in defined as a product of mass of an object and it acceleration.  

Acceleration is caused when speed changes over time.  

Among all the given options, in option D the speed over time will cause acceleration and thus produce a force.  

Force being a vector quantity required change of speed in  some direction. While running , both quantity and direction can be assumed to be changing.  

Thus, option D is correct.

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