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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.
(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.