Rather than matching definitions, we can test each statement against a simple, familiar scenario and see which one it actually explains.
Consider a hockey puck sliding across frictionless ice: it keeps moving at the same speed in the same direction until something, a wall, a stick, friction, pushes on it. That behaviour is described precisely by the statement that a body continues in its state of rest or uniform motion unless acted upon by an external force, the first option.
Now consider a rocket engine: firing it for longer or harder changes the rocket's momentum by a proportionally larger amount. That scenario is explained by the rate of change of momentum being proportional to the impressed force, which is the second law, not what is being asked about here.
And consider someone jumping off a small boat: the person moves forward while the boat is pushed backward. That paired, opposite-force behaviour is explained by the statement that to every action there is an equal and opposite reaction, the third law.
Since the question asks specifically about the first law, and the puck example (motion unchanged unless a force acts) is the one that matches it: \[ \boxed{\text{Every body continues in its state of rest or uniform motion in a straight line unless acted upon by an external force}} \]