Explanation:
When two current-carrying conductors are placed parallel to each other, the interaction between their magnetic fields determines their behavior. The direction of the magnetic field around a current-carrying conductor is given by the right-hand rule.

Parallel currents:
When the currents in both conductors flow in the same direction (parallel currents), their magnetic fields also have the same direction. This means that the magnetic field lines around each conductor are in the same direction and will interact with each other.

Magnetic field interaction:
The interaction between the magnetic fields of parallel currents can be understood using Ampere's law. According to Ampere's law, the magnetic field created by a current-carrying conductor is directly proportional to the current flowing through the conductor.

Attract:
When the magnetic field lines of the two parallel currents interact, they produce a net magnetic field that is stronger between the conductors. This results in an attractive force between the conductors.

Anti-parallel currents:
When the currents in both conductors flow in opposite directions (anti-parallel currents), their magnetic fields have opposite directions. This means that the magnetic field lines around each conductor are in opposite directions and will interact with each other.

Magnetic field interaction:
The interaction between the magnetic fields of anti-parallel currents can also be understood using Ampere's law. In this case, the magnetic fields produced by the two currents have opposite directions.

Repel:
When the magnetic field lines of the two anti-parallel currents interact, they produce a net magnetic field that is weaker between the conductors. This results in a repulsive force between the conductors.

Conclusion:
In summary, when two current-carrying conductors have parallel currents, their magnetic fields interact to create an attractive force between them. On the other hand, when the currents are anti-parallel, their magnetic fields interact to create a repulsive force between the conductors. Therefore, the correct answer is option B: attract, repel.