Answer
Telescope
Work Step by Step
We know that
$ d=f_1+f_2$
$\implies d=50.0cm+2.5cm=52.5cm $
We know that the distance between the two lenses is $52.5cm $; from the above calculation, we can see that the sum of the foal lengths of the two lenses is equal to the separation between the lenses; thus, the instrument is a telescope.