What Did the Supreme Court Justices Mean by the Principle

What did the Supreme Court justices mean by the principle "one person, one vote"?

A) Voters may only vote once in an election.
B) Within a state, electoral districts must have roughly equal populations.
C) No one could be denied suffrage on the basis of race or gender.
D) Voting was an individual right, not a group right.
E) Compulsory voting is constitutional.