The time period of a vibrating magnetometer is $T \propto 1/\sqrt{M}$. If the magnetic moment M decreases by 19%, the new moment is $M' = 0.81M$. The new time period $T' \propto 1/\sqrt{0.81M} = (1/0.9) * (1/\sqrt{M}) = T/0.9 ≈ 1.11T$. This is an increase of approximately 11%. Option (a) is the closest.