In the 12th question about which axis we have to find the angular velocity please give detailed explanation thank you

angular velocity for rigid body is same about any axis.now we know that in a uniform magnetic field (ie when net force of a body is zero) the torque is same about any axis. so as the sphere is in pure rolling the torque about any axis is same as found in question 11.so u can use conservation of enrgy to find the angualr velocity