Question

newtons third law of motion states that for every action, there is an equal and opposite reaction. it is also called the law of action-reaction.
an action force is a force acting in a specific direction. a reaction force is a force acting in the opposite direction as another force.
action-reaction forces act in pairs and are equal and opposite.
what is the best definition of an action force?
a force acting in a given direction on an object
the amount of matter in an object
when an object is at rest

Explanation:

The problem is about identifying the correct definition of an action force based on the provided text. The text clearly states that an action force is a force acting in a specific direction. Among the options, "a force acting in a given direction on an object" aligns with this definition. The second option ("the amount of matter in an object") is the definition of mass, not an action force. The third option ("when an object is at rest") is related to the state of an object (static equilibrium concept in physics but not the definition of action force).

Answer:

a force acting in a given direction on an object