Which of the following numbers has

Here we have to check terminating decimal expansion.

We know that if the numerator can be written in the form  where m and n are non negative positive integer then the fraction will surely terminate. We proceed as follows to explain the above statement

$\frac{21}{2^{3} \times 5^{6}}=\frac{2^{2} \times 21}{2^{2} \times 2^{3} \times 5^{3}}$

$=\frac{84}{(2 \times 5)^{5}}$

$=\frac{84}{10^{5}}$

$=\frac{84}{100000}$

$=0.00089$

Hence the correct option is (b).