Answer:
Option A.

Explanation:
In this figure side AB is a part of triangle formed diagonal of base of the box.
Side with length 1 unit and side AB.
Now we will calculate the length of diagonal of the base
=
[By Pythagoras theorem]
=

=

Now we apply Pythagoras Theorem again.
AB² = 1² + (
)²
= 1 + 52
= 53
AB =

Option A is the answer.