1. Oblivious
Who did Merlin think he was fooling?
Arthur wouldn't be a very good future king at all if he didn't notice Merlin's eyes changing color, his and Lancelot's shifty movements whenever their considerable luck in having branches/lighting fixtures fall in all the right places on missions was questioned, or the way Arthur was always conveniently unconscious whenever they were certainly about to die and woke up to find the threat gone.
He still couldn't believe Merlin thought 'you killed the dragon' was a passable lie. Dragons disintegrated into nothing when slain, did they?
So when Merlin finally plucked up the courage to tell Arthur, he just grinned and said, "Well, duh. I'm not oblivious, Merlin, now go heat up my bath."
Merlin's face was worth every lie.
