If the current direction is reversed in one of the strips, the magnetic field in a point A located outside the space between the strips, and very close to the upper strip (z << a) will be

Twice the initial value

Explanation:

Let the current be  = I

      the width of the conducting strips be = a

We know that magnetic field between two plates is given by

If the direction of this magnetic field is same between the two plates, then

  = 0

And when the currents runs opposite at each plate, then

Hence the magnetic field will be twice the initial value of the magnetic field that runs between the plates.

i think the answer is b