What indicates that you have a proper interval with the aircraft ahead of you?

A proper interval with the aircraft ahead is indicated when that aircraft is either abeam or behind your wingtip and has completed at least 90 degrees of a turn. This specific positioning demonstrates that there is enough horizontal separation between the two aircraft to ensure safety during flight operations.

When an aircraft is abeam or behind your wingtip, it confirms that there is adequate distance to prevent the possibility of collision or wake turbulence, which is crucial during phases like landing or taking off. The stipulation that the aircraft ahead has completed a minimum of 90 degrees of turn adds an additional layer of certainty, as it signifies that the aircraft is effectively transitioning to a different flight path that provides even more separation.

Other options do not fully capture the necessary conditions of separation and completion of the turn needed to ensure safety, underscoring why the chosen answer accurately reflects the requirements for maintaining a proper interval.