What is the definition of endogeneity in a regression model Y = β1X1 + ... + βpXp + ε?
1) There is an unobserved variable Z which is correlated with at least one of our predictors Xj
2) There is an unobserved variable Z which is correlated with all of our predictors X1, ..., and Xp
3) There exists an omitted variable Z which is correlated with Y
4) There exists a very high level of correlation between at least two predictors Xj and Xj' such that corr(Xj, Xj') > 0.9