Two parallel wires carry a current in the same direction. there is between the wires.

The correct answer is "an attractive force" between the wires.

Let's see why. Assume we have wire A on the left and wire B on the right, and that the current in both wires go upward.  First, let's find the direction of the magnetic field produced by wire A at wire B: by using the right-hand rule, we see that since the current (the thumb) goes upward, the magnetic field (given by the other fingers) at wire B is directed inside the paper.
Then we can apply again the right-hand rule to see what is the force on wire B. The index gives the direction of the current (upward), the middle finger the direction of the magnetic field (inside the paper), and the thumb gives the direction of the force: to the left, so toward wire A. This means the force is attractive. (you can re-do the procedure on wire A, and you will find the force on wire A is directed toward wire B)