c
\(\begin{array}{l}
Here\,T = \frac{1}{2}\sec \,the\,required\,centripetal\\
acceleration\,for\,moving\,in\,a\,circle\,is\\
{a_C} = \frac{{{v^2}}}{r} = \frac{{{{\left( {r\omega } \right)}^2}}}{r} = r{\omega ^2}r \times {\left( {2\pi /T} \right)^2}\\
so\,{a_c} = 0.25 \times {\left( {2\pi /0.5} \right)^2}\\
\,\,\,\,\,\,\,\,\,\,\, = 16{\pi ^2} \times .25 = 4.0{\pi ^2}
\end{array}\)