Correct Answer - A
When restoring force will become equal to the frictional force, block will start to slip.
`therefore` Restoring force = Friction force
`rArr" "kA = mu mg" "...(i)`
Frequency, `f = (1)/(2 pi)sqrt((k)/(m))"and from Eq. (i)"f = (1)/(2pi) sqrt((mug)/(A))`