For chemosynthesis to support life on Europa, compounds that could indicate potential metabolic processes would typically include hydrogen peroxide and methane. However, among the options provided, oxygen is often a significant indicator of life as it can be a byproduct of photosynthesis or respiration in aerobic organisms.
Given the context of Europa and its subsurface ocean, if you are looking for what would most directly indicate that chemosynthesis is occurring, methane would be relevant because it can be produced by methanogenic organisms via chemosynthetic processes.
In summary, while all of the suggested compounds could be relevant in different contexts, methane is a key compound that might suggest biological processes related to chemosynthesis.