Which of the Following Assumptions Must Be Made in Order $\sigma$

Multiple Choice

Which of the following assumptions must be made in order to use the pooled least squares estimator,but is relaxed in the cluster robust model?

A) E(e_it) = 0
B) var(e_it) = E(e²_it) = $\sigma$ ²
C) cov(e_it,e_js) = E(e_it,e_js) = 0 for i j or t s
D) cov(e_it,x_2it) = 0,cov(e_it,x_3it)