b
\(\text { Given : } R=9 \mathrm{~m},\)

\(120 \text { revolution in } 3 \mathrm{~min}\)

\(\omega=\frac{120 \mathrm{Rev}}{3 \mathrm{~min} .}=\frac{120 \times 2 \pi \mathrm{rad}}{3 \times 60 \mathrm{sec}}=\frac{4 \pi}{3} \mathrm{rad} / \mathrm{s}\)

\(\mathrm{a}_{\text {centripetal }}=\omega^2 \mathrm{R}=\left(\frac{4 \pi}{3}\right)^2 \times 9=16 \pi^2 \mathrm{~m} / \mathrm{s}^2\)