How can a player play a land?

A player can play a land during their main phase while they have priority and the stack is empty. This is because the rules specify that lands can only be played during the main phase of a player's turn and only when that player has priority. A player cannot play a land while an opponent has priority or during any phase other than their main phase.

Playing a land requires that the player be able to act without any other spell or ability being on the stack. This is why it is essential for the stack to be empty at the time of playing a land. The restriction to the main phase ensures that players can't just drop lands at any point in the game, adding strategic elements to gameplay.

The other options reflect misunderstandings about the timing and conditions under which lands can be played in Magic: The Gathering. For instance, a player cannot play a land during their opponent's turn or during phases other than their main phase, and having lands in hand doesn't automatically mean they can play them at any time. Thus, the correct answer emphasizes the importance of the phase, priority, and the stack in land play.