Pollsters Are Likely to Use Weighting in Which of the Following

Pollsters are likely to use weighting in which of the following situations?

A) when observed results need to be adjusted according to what is known about specific proportions from the larger population
B) when observed results are equivalent to what is known about specific proportions from the larger population
C) in all randomly selected sampling situations involving large populations of American citizens
D) this technique is rarely used, as it carries with it a set of sampling errors and other biases