Without any other context, either option can indeed be defended. After all, whose decision making process is being questioned, that of the entire jury (singular) or that of individual members? If you think it's the former, then the answer is "its." If you think it's the latter, then the answer is "their". It's easier to see this distinction if you replace "some of the jurors" with something singular...
Case 1: "After the jury gave the decision that the defendant was guilty, Jonathan, one of the jurors, started questioning his decision making process." - in this case, Jonathan is questioning his own decision making process, not that of the jury as a whole.... arguably confusing (because the sentence refers to both the decision of the jury and the decision making process of one of the jurors) but not incorrect
Case 2: "After the jury gave the decision that the defendant was guilty, Jonathan, one of the jurors, started questioning its decision making process." - in this case, Jonathan is questioning the decision making process of the jury
If we go with case 1, but replace Jonathan with something plural, then the possessive pronoun his must be changed to their, as in the posted sentence. But if we go with case 2, its will remain unchanged.
So how do we know which to use? As presented in the post, the answer to this question depends on your interpretation of the sentence. Luckily, on the GMAT, other factors and clues will lead us to the best answer. Unfortunately, the full answer choices to this question were not included in the post. If we trust the "correct" answer, then we can defend it and hope the actual answer choices contained other supporting evidence; otherwise, the answer is debatable. Either way, official GMAT questions will not be this ambiguous, so don't worry too much about this one if you are still confused!