Significant Digits
Here are the rules to determine the significant digits of a number
* Non-zero digits are always significant
* Any zeros between two significant digits are significant.
* A final zero or trailing zeros in the decimal portion ONLY are significant.
The given number is:
If the number is expressed in scientific notation, the coefficient to consider is 1.591 and it has 4 significant digits.
But if we express the number in the decimal form we get 1,591,000. In this case, all the digits (7) are significant, including the zeros because they are part of the decimal portion.
From the two possible answers, I'd select the first one: 4 significant digits, since the number is expressed in scientific notation.