b
In normal adjustment, object lies close to focus of objective and image is formed at infinity, magnification, \(\mathrm{m}=\frac{\mathrm{Ld}}{\mathrm{f}_{\mathrm{e}} \mathrm{f}_{0}}.\)

Where \(f_{0}=1.6 \mathrm{\,cm}, f_{\mathrm{e}}=2.5 \mathrm{\,cm}, \mathrm{d}=21.7 \mathrm{\,cm}\)

\(\mathrm{L}=\mathrm{d}-\mathrm{f}_{0}=21.7-1.6=20.1 \mathrm{\,cm}\) ( approx. )

\(\Rightarrow \mathrm{m}=\frac{21.7 \times 20.1}{1.6 \times 2.5}=\frac{436.17}{4}=109.1 \simeq 110\)