The concentration of ddt (c14h9cl5), in milligrams per liter, is:
(1) a nominal variable
(2) an ordinal variable
(3) an interval variable
(4) a ratio variable.

The correct option is 4) a ratio variable.

Step-by-step 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.

c. (3,2,6) is the answer.