Out of the Answer Choices that you gave, "6" would be corect.
Answer: 6.4To solve this problem we are going to be using the
Pythagorean Theorem.
Pythagorean Theorem:

I Graphed your points. Now we will find a and b.
Now we will fill in the formula [look at picture to find where i got my numbers]
a^2 + b^2 = c^2
4^2 + 5^2 = c^2
16 + 25 = c^2
41 = c^2
6.4 = c
Sorry for the bad Picture. I drew the triangle myself.