Answer:
√37
Step-by-step explanation:
An irrational number is a number that never terminates nor repeats. 
Let's go through each of the answer choices. 
1) √100 is just 10. This is not irrational.
2) √81 is just 9. This is again not irrational. 
3) √64 is 8. Not irrational. 
4) √49 is 7. Again, not irrational. 
5) √37 is 6.08276253... This number will never end and it doesn't repeat. Therefore, √37 is irrational. 
And this is our answer. 
And we're done!