d
\(\begin{array}{l}
{\rm{Using,}}\,\mu {\rm{mg = }}\frac{{m{v^2}}}{r} = mr{\omega ^2}\\
\omega  = 2\pi n = 2\pi  \times 3.5 = 7\pi rad/\sec \\
Radius,\,r = 1.25\,cm = 1.25 \times {10^{ - 2}}m\\
Coefficient\,of\,friction,\,\mu  = ?\\
\mu mg = \frac{{m{{\left( {r\omega } \right)}^2}}}{r}\left( {v = r\omega } \right)
\end{array}\)

\(\begin{array}{l}
 \Rightarrow \mu  = \frac{{r{\omega ^2}}}{g} = \frac{{1.25 \times {{10}^{ - 2}} \times {{\left( {7 \times \frac{{22}}{7}} \right)}^2}}}{{10}}\\
\,\,\,\,\,\,\,\, = \frac{{1.25 \times {{10}^{ - 2}} \times {{22}^2}}}{{10}} = 0.6
\end{array}\)