c
(c) Maximum force by surface when friction works

\(F = \sqrt {{f^2} + {R^2}} = \sqrt {{{(\mu R)}^2} + {R^2}} = R\sqrt {{\mu ^2} + 1} \)

Minimum force \( = R\) when there is no friction Hence ranging from \(R\) to \(R\sqrt {{\mu ^2} + 1} \)

We get, \(Mg \le F \le Mg\sqrt {{\mu ^2} + 1} \)