The time period of a vibrating magnetometer is $T \propto 1/\sqrt{M}$. If M decreases by 19%, the new moment is $M' = 0.81M$. The new time period $T' \propto 1/\sqrt{0.81M} = (1/0.9) \times (1/\sqrt{M}) = (10/9)T \approx 1.11T$. This is an increase of about 11%. Option (a) is the closest. [cite: 1, 1405, 1407]