The figure below shows a block of mass $M$ connected to an ideal string which passes through a thin fixed smooth pipe. On the other end, a particle of mass $m$ is connected which revolves in a vertical circle of radius $r$. If the coefficient of friction between $M$ and the surface is $\mu =\frac{2}{3}$, then for what minimum value of $M$, the block of mass $m$ can undergo complete vertical circular motion?

• A

  $M_{\min }=6m$
• Correct answer
  B

  $M_{\min }=9m$
• C

  $M_{\min }=3m$
• D

  $M_{\min }=15m$

Hence, the correct answer is option: 'B'.