The number of significant zeros present in the measured value 0.020040 is a) 5 b)2 c)1 d)3

Basic rules for significant digits:

1)  All nonzero digits are significant.
2)  All zeroes between significant digits are significant.
3)  All zeroes which are both to the right of the decimal point and to the right of all non-zero significant digits are        themselves significant.

So, in the given question 3rd rule will be followed. And there are 3 zeroes to the right of both the decimal point and all non-zero significant digits.