Answer:
The correct option is 4) a ratio variable.
Explanation:
Consider the provided information.
Nominal variables are pertaining to names or It merely name differences, it is a qualitative variables.
Ordinal variable: It is a rank-order observations in which order matters but difference between the value doesn't matters. It is a qualitative variables.
Interval variable: It is useful if the difference between two values is meaningful. It is a quantitative variables.
Ratio variable: this variable has all the properties of an interval variable, also it has a clear definition of 0.0. It is a quantitative variables.
Now consider the provided information.
The concentration is in milligrams per liter which is a quantitative variable.
Among the provided options only ratio variable and interval variable is quantitative variable. So option A and B are incorrect.
Since the milligrams per liter can be zero point which is not the characteristic of interval scale. Thus, the option C is incorrect.
The zero point is characteristic of ratio variable. Thus, the concentration of DDT (C14H9Cl5), in milligrams per liter, is ratio variable.
Hence, the correct option is 4) a ratio variable.