'Lock the barn door after the horse has bolted' (also 'lock the stable door...') is a proverb about taking preventive measures too late - after the damage has already occurred or the opportunity lost. The horse has already escaped, so locking the door now is futile. It means being careful or trying to make something safe when it is already too late to prevent the negative outcome. It does not mean acting at home (B), becoming angry (C), or wasting time (D).