The correct answer is - C) keep the Earth from tilting on its axis.
The difference in the length of the days in different parts of the year, appear because of the tilting of the Earth on its axis. By tilting, the Earth provides one part of the planet with more sunlight, while the other one with less, in one period of the year, while in the other half of the year, the roles will be reversed.
If somehow, it is possible to stop the Earth from tilting, than all of the days on the planet, throughout all of the year, will have the same length. It has to be said though, that such thing will dramatically change pretty much everything on the planet.