Answer: Option C.
Explanation:
The lengths of our triangle are:
9, 10 and √130.
If the triangle is a triangle rectangle, by the Pitagoream's theorem we have:
A^2 + B^2 = H^2
in this case H is the larger side, this must be √130.
then:
A and B must be 9 and 10.
9^2 + 10^2 = (√130)^2
81 + 100 = 130
This is false, so this is NOT a triangle rectangle, the hypotenuse is shorter than it should be.
Now, we have some kind of rule:
if A^2 + B^2 = H^2 then we have one angle of 90° and two smaller ones. (triangle rectangle)
if A^2 + B^2 > H^2 then all the angles are smaller than 90°, this is an acute triangle.
if A^2 + B^2 < H^2 then we have one angle larger than 90°, this is an obtuse angle.
(H is always the larger side, A and B are the two shorter ones).
In this case:
81 + 100 > 130
Then this must be an acute angle.