#### Answer

a). $(2)$ one with shorter focal length
b). $-280X$ and $-360X$

#### Work Step by Step

a). (2) one with the shorter focal length should be used as objective since more is the focal length less is the total magnification of microscope as m is inversely proportional to the focal length.
since, $m_{total}=-\frac{25(cm)L}{fofe}$
b). $m_{total}=-\frac{25(cm)L}{fofe}$
For lens of focal length 0.35cm, $m_{total}=-\frac{25\times 15}{0.35\times3}=-360X$
For lens of focal length 0.45cm, $m_{total}=-\frac{25\times 15}{0.45\times 3}=-280X$