b
(b) Whenfinal image is formed at \(\mathrm{D}=25 \mathrm{cm}\) from eye. In this situation, \(\mathrm{v}=-\mathrm{D}\) from lens formula, \(\frac{1}{v}-\frac{1}{u}=\frac{1}{f^{\prime}}\)
we have, \(\frac{1}{-D}-\frac{1}{-u}=\frac{1}{f}\)
i.e., \(\frac{D}{u}=1+\frac{D}{f}\)
So magnifying power \(=\frac{D}{u}=\left(1+\frac{D}{f}\right)\)