The discovery that would make the existence of extremophiles in Europa's ocean less plausible is the ocean lacking methane or hydrogen sulfide.
Methane and hydrogen sulfide are often associated with biological processes on Earth, particularly in extreme environments where extremophiles thrive, such as hydrothermal vents. If Europa's ocean lacks these compounds, it could indicate a less favorable environment for the life forms typically found in extreme conditions.
In contrast, the other options could still allow for the possibility of extremophiles: acidic or basic conditions might still support certain life forms, ice covering the ocean doesn’t rule out life because many extremophiles live in lightless conditions, and high salinity might also support some unique extremophiles adapted to such environments.