JEE Main 2015 · 4 Apr · Shift 1, Physics question 19

The inner solenoid's field is confined inside it, so it exerts no force on the outer solenoid: $\vec F_2=0$.

Over the inner solenoid, the outer one's field $\vec B$ is uniform and axial. Each element of the inner winding carries current $I\,d\vec l$ round the axis, so $I\,d\vec l\times\vec B$ points radially. With both currents in the same sense, $\hat\phi\times\hat z=\hat r$: the force is radially outwards on every element. (These outward forces add to zero as a vector, but the question asks for their direction.)