As soon as Macbeth commits murder, the weight of his conscience becomes almost palpable. This is the only suitable word that can be used here - palpable means that something is so strong and intense that it can almost be touched. So, when Macbeth killed his first victim, he became really anxious and guilt-ridden that it could be felt without a problem. Stealthy means hidden, multitudinous means numerous, and augmented means larger, but not in this context, which is why these are the incorrect options, and only palpable is correct.