Note: You forgot to add the diagram, so I found the missing diagram and attached below, based on which I am solving your query. Hopefully, it would clear your concepts anyways.
Answer:
Option B is correct, as
Explanation:
From the attached figure,
Δ
is a right triangle.
Applying Pythagoras theorem,



switching sides



Note that Δ
s also right triangle.

∵




But length of a side can not be negative, so ignore the negative solution.
so

∵

Therefore, option B is correct, as