Answer:
a
Explanation:
For this question we need to use the Pythogorean Theorem as we know its a right angle and are missing a side.
The Pythogorean Theorem is
, where a and b are any side other than the hypothenuse, the longest line, and c is the hypothenue.
So, plug in the values and you get


So now we just find the square root, which as it isn't a square number we can just leave it as it is, as a surd, so the answer is 'a'.