The correct option is $A$ Frequency of driving force
In forced oscillation,we provide energy at the mean position, which helps the body to oscillate at a constant amplitude.It basically compensate the loss of energy which occur due to surrounding energy loss. A particle oscillates simple harmonically with a frequency equal to the frequency of driving force in steady state.
$x = a\,\sin \,\left( {\omega t + \pi /6} \right)$
After the elapse of what fraction of the time period the velocity of the particle will be equal to half of its maximum velocity?