Answer:
Options b and c should be highlighted.
Explanation:
Non-exact square roots are irrational. For example, the square root of 2, 3, 5, 6, 7...
Exact square roots are rational, for example, the square root of 1, 4, 9, 16, 25, ...
In this question:
The square root of 81 is 9. So this is rational.
57 does not have a exact square root. So the square root of 57 is irrational. The same goes for 21.
The square root of 100 is 10, so this is rational.
Options b and c should be highlighted.