What Condition Best Describes the Endogeneity Problem

Multiple Choice

What condition best describes the endogeneity problem?

A) The variance of the errors (U_i) depends on X_i.
B) Some variables within X_i are perfectly correlated with other variables in X_i.
C) The distribution of the errors (U_i) is non-normal.
D) One of the X_i variables is correlated with the error term (U_i) .