a

Time Period of oscillating magnet is given bt $ T = 2\pi \sqrt {\frac{I}{MB}}$

Where $ I = \frac{ML^2}{12}$

When the magnet is broken in two equal parts, the new magnetic dipole moment is 

Mass of broken part $M' = \frac{M}{2}$

Length of broken part $L' = \frac{L}{2}$

Moment of Inertia of broken part $I' = \frac{M'L'^2}{12}=\frac{ML^2}{96}=\frac{I}{8}$

Ratio of time Period $\frac{T'}{T} = \sqrt{\frac{I'}{M'}\times\frac{M}{I}}=\sqrt{\frac{I'}{I}\times\frac{M}{M'}}=\sqrt{\frac{1}{8}\times\frac{2}{1}} =\frac{1}{2} $

$\Rightarrow T'= 2 s$