Tom Bradley was mayor of Los Angeles from 1973 to 1993, an era when the city had transformed from a collection of suburban neighborhoods to the second-largest city in the United States.
(A) an era when the city had transformed
(B) an era during which the city was transformed
(C) an era that transformed it
(D) during which era the city transformed
(E) during which the city was transformed

I want to understand what is the rule regarding the word during
why we can say during which in D and E but can say in B

thanks