d
(d) \(\frac{1}{f} = \left( {\frac{{{n_2}}}{{{n_1}}} - 1} \right)\,\,\left( {\frac{1}{{{R_1}}} - \frac{1}{{{R_2}}}} \right)\)

where \({n_2}\) and \({n_1}\) are the refractive indices of the material of the lens and of the surroundings respectively. For a double concave lens,

\(\left( {\frac{1}{{{R_1}}} - \frac{1}{{{R_2}}}} \right)\) is always negative.

Hence \(f\) is negative only when \({n_2} > {n_1}\)