Which one of the following statements is not true?

Correct option is B

(a) √64 = 8: This is true because 8 x 8 = 64.
(b) √0.4 = 0.2: This is false. If you square 0.2, you get 0.04, not 0.4.
(c) 0 is a rational number: This is true. A rational number is any number that can be expressed as a fraction of two integers. 0 can be expressed as 0/1, which is a fraction.
(d) 243 is not a perfect cube: This is true. A perfect cube is a number that can be expressed as the cube of another integer. The cube root of 243 is approximately 6.24, which is not an integer.