What Makes Europa the Most Likely Place Extraterrestrial Life Will

What makes Europa the most likely place extraterrestrial life will be found in our solar system?

A) It has an ocean of liquid water,which is warmed by tidal force interaction with Jupiter,and may house underwater volcanic thermal vents.
B) It has an ocean of liquid water,and there is increasing evidence of phytoplankton activity,particularly as shown by ongoing spectral analysis.
C) It has a hydrogen ocean and moderate levels of oxygen in its atmosphere,which is gradually producing water through frequent lighting strikes.
D) Actually,Io is the most likely place to find life,due to its high level of volcanic activity.