What numbers does the square root of 355 fall between?
a)15 and 16
b)17 and 18
c)18 and 19
d)20 and 21

The answer is c)18 and 19.

Let's go through all choices:
a) 15 and 16 is not correct because √355 > 16 > 15: 
15 = √15² = √225
16 = √16² = √256
√355 > √256 > √225   ⇒ √355 > 16 > 15

b) 17 and 18 is not correct because √355 > 18 > 17: 
17 = √17² = √289
18 = √18² = √324
√355 > √324 > √289   ⇒ 19 > √355 > 18

c) 15 and 16 is correct because : 
18 = √18² = √324
19 = √19² = √361
√361 > √355 > √324  ⇒ 19 > √355 > 18

d) 20 and 21 is not correct because 21 > 20 > √355: 
20 = √20² = √400
21 = √21² = √441
√441> √400> √355 ⇒ 21 > 20 > √355