Consider option (a).
2 + √
4 can be simplified as 2 + 2 = 4 which is a rational number.
Therefore, this option is not correct.
Consider option (b).
√
20 + 5 can be simplified as √
25 = 5 which is a rational number.
Therefore, this option is not correct.
Consider option (c).
3 + √
21 cannot be simplified further.
So, it is an irrational number.
Therefore, this option is correct.
Consider option (d).
4 + √
25 can be simplified as 4 + 5 = 9 which is a rational number.
Therefore, this option is not correct.
