Question

A force F is applied at the centre of disc of mass M .the minimum value of coefficient of friction of the surface for rolling is

Answer 1

Torque applied due to external force
т₁ = FR
F is the force applied and R is the radius of the disk

Normal force on the disc
N = Mg
Frictional force on the disc
f = μ Mg

Torque due to friction
т₂ = μMgR

Condition for rolling 
Torque on the ball due to friction should not be less than torque due to external force.
т₂ ≥ т₁
μMgR ≥ FR
μMg ≥ F
μ ≥ F ÷ Mg

Therefore the minimum value of coefficient of friction of rolling is F ÷ Mg. 

Answer 2

See the attached file the answer is right

That girl who answered first had done wrong ans is F/3Mg

And not F/Mg