Hi mokkyukkyu,
Thanks for your question!
Our job is not to question the argument, but identify the function of a particular argument part. To answer your question, the author most likely used the word "warm" to contrast the water temperature to that of the icy surface, which is obviously cold. The argument is certainly not bullet-proof: the buckled icy surface could have easily been caused by something else, or the water temperature below may still be too cold to support life.
None of this has any bearing on the structure of the argument, which is key to attacking this question.
The argument is structured as follows:
Premise: Photographic evidence of a buckled icy surface.
Sub. Conclusion: There is a warm sea beneath the surface.
Premise: The presence of such a sea is the primary factor in the early development of life.
Conclusion: There may be life on Europa.
Since the referenced claim is at once supported by the first sentence, and also supports the last sentence in the stimulus, it is a subsidiary conclusion - validating answer choice (A).
Hope this clears it up!
Thanks,