Let us divide the top semi-cylindirical shell into small strips of width dw so that each divided strip behaves like wire of infinite length.
Let us assume the current i1 distributed uniformly and the current density α per unit width is given by
magnetic field at center due to a single strip is
Hence the magnetic fiel at center due to total current flowing in top semi-cylindrical is
Since the current of same magnitude i1 flows but in opposite direction in bottom seni-cylindrical shell,
same magnetic field as calculated above is generated at center as calculated above.
Hence total field at center due to current flowing in both semi-cylindrical shell is
Force per unit length on wire is B×i2×1 =
I did not get any answer given in the options, but i believe this is the way to solve this problem