A reliable method for antonym questions is to build the opposite word yourself before checking the choices. Compassionate is rooted in "sympathy," so its natural opposite would be something like "unsympathetic" or "uncaring." Holding this self generated opposite in mind, we scan the options for the closest match.
"Unsympathetic" appears directly among the choices and matches the self generated opposite almost exactly, since it explicitly negates the sympathy that defines compassion.
Testing the distractors against this same benchmark: indecisive relates to decision making, unlawful relates to legality, and untrustworthy relates to honesty and reliability. None of these touch upon sympathy or emotional concern for others, confirming they are unrelated traits included only to distract.
Let's summarize:
Pre forming your own antonym before reading the choices is a strong strategy when the options include several unrelated negative sounding words.