To check answer A and B, we have to know the fact that
3 angles in a triangle add up to 180 degrees.
Let's check:
A)

Thus, this is a triangle.
B)

This is also a triangle.
To check answer C and D, we have to know the triangle inequality theorem.
The triangle inequality theorem tells us that the sum of any two sides of a triangle is always greater than the third side.
Let's check:
C)
5.5, 5.5, and 11 are the sides.
Immediately we can check:
5.5 + 5.5 ?> 11
11 > 11 --- NO!
Thus, the triangle inequality theorem doesn't hold here. The sum of any two sides has to be greater than the third side.
This is not a triangle.
Correct Answer
C