The correct option is D Future Perfect Continuous
Here, 'will have been sailing' indicates Future Perfect Continuous. It is
used to indicate an action that will have happened for some time and will not
be complete yet at a certain point in the future.
Present Perfect Continuous (have been sailing) is
used to indicate an action that started at some point in the past and may or
may not be complete.
Future Perfect (will have sailed) is
used to indicate an action that will be complete before another event takes
place.
Future Continuous (will be sailing) indicates
what will be going on in the near future.