What Is the "Third-Variable Problem

What is the "third-variable problem"?

A) When a researcher measures so many variables that it's difficult to make sense of the data.
B) When a research considers only one direction of causation between two variables that are strongly correlated.
C) When a researcher measures variables that are not relevant to the relationship he or she is most interested in.
D) When a researcher fails to consider that two variables may be correlated due only to the effects of some unmeasured third variable.