Final answer:
The number of significant figures in the decimal number 2.0×10⁻⁴ is four. This is determined by counting all non-zero digits and zeroes that separate two non-zero digits. In this case, all four digits after the decimal point are significant because they separate two non-zero digits, making the total number of significant figures four. Thus the correct option is (d).
Step-by-step explanation:
When we write a number with a decimal point, the significant figures are the digits that contribute to the measurement being made. In this case, we are dealing with a very small number, so all four digits are significant. The "2" and the "0" before the decimal point are not significant because they are multiplied by 10^4, which is a large number that does not contribute significantly to the measurement being made. Therefore, we only count the digits after the decimal point as significant figures. Thus the correct option is (d).
In general, when determining the number of significant figures in a number, we follow these rules:
1. All non-zero digits are significant.
2. Zeroes between non-zero digits are significant.
3. Zeroes at the end of a number (to the right of the decimal point) may or may not be significant, depending on whether they separate two non-zero digits or not. If they do, then they are significant; if they don't, then they are not.
In our example, all four digits after the decimal point are significant because they separate two non-zero digits. Therefore, we have 4 significant figures in 2.0×10⁻⁴.