d
(d)In the sum and difference method of vibration magnetometer \(\frac{{{M_1}}}{{{M_2}}} = \frac{{T_2^2 + T_1^2}}{{T_2^2 - T_1^2}}\) Here \({T_1} = \frac{1}{{{n_1}}} = \frac{{60}}{{12}} = 5\sec .\;{T_2} = \frac{1}{{{n_2}}} = \frac{{60}}{4} = 15\sec \) \(\therefore \frac{{{M_1}}}{{{M_2}}} = \frac{{{{15}^2} + {5^2}}}{{{{15}^2} - {5^2}}} = \frac{{225 + 25}}{{225 - 25}} = \frac{5}{4}\)