Force between the two parallel wires carrying currents has been used to define.
(A) Ampere (B) Coulomb (C)Volt (D)Watt

Hint:In the formula for calculating the force per unit length between two straight ,parallel current carrying wires, put the current through both the wires as unity. Also consider the distance between the wires as unity.
Step-by-step Solution:
We know that force per unit length between two parallel current carrying wires is given by- $\dfrac{F}{l} = \dfrac{{2\mu {I_1}{I_2}}}{{4\pi r}}$
where ${I_1}$=current flowing through conductor 1. $r$=distance between the conductors 1 and 2.
${I_2}$=current flowing through conductor 2.

Now, putting ${I_1}$=${I_2}$=1A, and putting the distance between them $r$=1m ,we get- $\dfrac{F}{l} = \dfrac{\mu }{{4\pi }} \times 2 \times 1 \times 1$
We know that, $\dfrac{\mu }{{4\pi }} = 2 \times {10^{ - 7}}$ ,putting this value in above equation we get-
$\dfrac{F}{l} = 2 \times {10^{ - 7}}$ N/m
From the above calculations ,it is clear that the force between the two parallel current carrying wire is $2 \times {10^{ - 7}}$N/m ,if the current between the wires is 1A(ampere). We can conclude that the force between two parallel current carrying wires is used to define ampere.

Hence, option (A) is correct.

ADDITIONAL INFORMATION:If the currents in both the wires flow in opposite directions ,the wires will move away from each other. And if the currents in both the wires flow in the same direction, the wires will attract each other.

The force between the wires and the distance between them vary inversely with each other i.e. if we double the distance between them ,the force becomes half. Similarly if half the distance between them ,the force becomes double.

Note-The ampere is that value of the steady current which, when maintained in each of the two long ,straight wires placed 1m apart ,would produce a force $2 \times {10^{ - 7}}$,on each other.