Which of the following is smallest?

Correct option is D

Solution:
We will calculate the value of each expression approximately:
 √9 − √7 = 3 − 2.645 = 0.355
 √6 − √4 = 2.449 − 2 = 0.449
√28 − 5 = 5.291 − 5 = 0.291
√18 − 4 = 4.243 − 4 = 0.243
Among all options, option (d) gives the smallest value.