What should happen if the jack is unmarked and becomes displaced during play?

When the jack is unmarked and becomes displaced during play, the correct action involves the authority of the opponent to reposition the jack. This rule is grounded in the principle that if the jack is not marked (which means it hasn't been designated with a marker or identified in a way that its original position is confirmed), it lacks a definitive position on the green. Consequently, the rules provide that the opposing player is given the power to place the jack back in a position they believe to be its original spot before it was displaced.

This process ensures fairness in the game, as a displaced jack can affect the strategy and outcome of the end. The repositioning must aim to accommodate the game's integrity, taking into consideration the usual markers such as nearby bowls or natural indicators on the green to guide the placement.

The other choices aren't appropriate in this scenario as they either undermine the necessary player dynamics in repositioning the jack or misinterpret the rules regarding unmarked jacks during play.