Why Do Presidential Candidates Tend to Focus Their Efforts on Battleground

Why do presidential candidates tend to focus their efforts on battleground states?

A) Battleground states have the most Electoral College votes.-Consider This: A handful of states will decide the election result.
B) Battleground states have more electors than they would deserve if electors were allocated by population.
C) Candidates focus on battleground states in order to increase their media exposure.
D) The winner-take-all system makes battleground states more relevant to a campaign.