I think it's between C and D. C is just a stronger version of D.
Like usual, we simplify:
(i) An earth-like planet is discovered.
(ii) The planet is in another solar system.
(iii) --------------------
(iv) Thus, greatest chance of extra terrestrial life is outside our solar system.
(iii) has to link the existence of life with an earth-like planet or conditions. The assumption, then, is: we discovered an earth-like planet and we have discovered life!
What if the planet is earth-like in conditions/appearance only and life has never been introduced there before?
(C) and (D) both say the same thing. You'll choose (C) over (D) because we're talking about a whole planet and not just conditions. You'll choose (D) over (C) because (C) talks in absolute terms ("always") while (D) tones it down and uses likelihoods.
If I was a GMAT testmaker, I wouldn't subject the students to the agony of this judgement call and change one thing in either of the choices to making it lacking in some respect.
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