Which of the Following Legal Principles Must Be Followed in Redistricting

Which of the following legal principles must be followed in redistricting legislative districts?

A) the population of the districts must be as equal as is possible
B) the district lines must be redrawn after each census to reflect the election returns
C) districts may not be gerrymandered to assist one political party's efforts to win election
D) districts may not be changed except with court permission