#### Answer

a). (2) increases the length of the telescope
b). $m=8X$
c). $L=85cm$

#### Work Step by Step

a). Given a terrestrial telescope has three lenses and one of them is erecting lens.
Erecting lens (2) increases the physical length of the telescope because according to construction of terrestrial telescope the object for this lens is at twice its focal length. Hence the image formed is neither magnified nor reduced.
Thus erecting lens increases the length of the telescope by $4fi$.
b). $m=-\frac{fo}{fe}=-\frac{40}{5}=-8X$
As the final image is upright hence $m=8X$
c). Length of telescope barrel $=fo+4fi+fe=40+4+5=85cm$