Which One of the Following Statements Concerning the Proper Time

Which one of the following statements concerning the proper time interval between two events is true?

A) The proper time interval is the longest time interval that any inertial observer can measure for the event.
B) The proper time interval is the shortest time interval that any inertial observer can measure for the event.
C) The proper time interval is the time measured by an observer who is in motion with respect to the event.
D) The proper time interval depends upon the speed of the observer.
E) The proper time interval depends upon the choice of reference frame.