Lowkey mad that all Arisa could think of for liking Mari was her face but whatever
When she answers, Arisa is trying to think back to what it was, and her "first" memory is of waking up in the hospital after getting amnesia and seeing Mari's face. But at that point she already subconsciously "knows" she loves Mari, so it's "love at first sight, take 2."
Even if we read this at face value, though, it is the exact same answer that Sayaka (who is no superficial idiot) gave about Touko in YagaKimi.