Which Assumption Is Most Likely to Be Violated with Times $\sigma$

Multiple Choice

Which assumption is most likely to be violated with times series data:

A) E(e_t) =0
B) var (e_t) = $\sigma$ ²
C) cov(e_t,e_s) =0,t≠s
D) e_t N(0, $\sigma$ ²)