Which of the following is an example of a ratio variable?

Enzyme activity is considered a ratio variable because it possesses all the characteristics of a ratio scale. Ratio variables have a true zero point, which indicates a complete absence of the measured quantity. In the case of enzyme activity, a measurement of zero reflects the complete absence of enzyme activity. Furthermore, ratio variables allow for meaningful comparisons between values. For instance, if one sample of enzyme activity measures twice that of another, it can be interpreted quantitatively how much more active one is than the other.

Other options, while they may be measurable, do not fully meet the criteria of a ratio variable. For example, temperature in Fahrenheit, while it has gradation, does not have a true zero point in the same sense, as 0°F does not indicate the absence of temperature. Similarly, SAT scores and credit scores are ordinal in nature; they allow for ordering but do not have a true zero point that represents the absence of the attribute being measured.