The force between two current-carrying parallel wires has been used to define
A) Ampere
B) Coulomb
C) Volt
D) Watt

Hint: The force per unit length between two current-carrying wires depends on the product of the current flowing in the individual wires and inversely proportional to the distance between them. Ampere is the unit of current.
Formula used: In this solution, we will use the following formula:
Force per length between two current-carrying wires: $\dfrac{F}{l} = \dfrac{{2{\mu _0}{I_1}{I_2}}}{r}$ where ${I_1}\,{\text{and}}\,{I_2}$ are the current in the two wires, $r$ is the distance between them.

Complete step by step answer:
We know that the force per length between two current-carrying wires is given by:
$\dfrac{F}{l} = \dfrac{{2{\mu _0}{I_1}{I_2}}}{r}$
In this equation, for two given wires, we can calculate the force that acts between them and we can also measure the distance between the two wires experimentally. Then we can say that 1 Newton of force acts per unit length for two current-carrying wires that carry a current of 1 Ampere and are placed 1 metre apart.
Hence the force between two current-carrying parallel wires is used to define the units of Ampere.

So, option (A) is the correct choice.

Note: The force between two current-carrying wires depends on the direction of the currents in the two wires. If the currents flow in opposite directions, the force is attractive and if the current is in the same direction, the force is repulsive. One ampere of current corresponds to the value carried by two wires which when placed one metre apart will experience a force of 1 Newton between each other.