Correct Answer - A
Magnification for parallel rays,
`m=(f_(v))/(f_(e))Rightarrow 20=(80)/(f_(e)) or f_(e)=40cm`
If the focal length of erecting lens is 20cm, then the length of the telescope
`L_(v)=f_(v)+4f+f_(v)`(where f is the focal length of erecting lens)
`=80+4xx20+4`
=80+80+4=164cm