All are perfectly fine, but a. is the better choice because it directly answers the question "Are you feeling alright? (i.e., how's your health)." Speakers often use that question as a way of politely asking if everything is OK. That is, even if you know the person isn't ill, it's more polite to ask about their physical health than to ask about their emotional state of mind (i.e., mental health). If it's the person's emotional state that's in question, one might use "Is everything OK?"
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