Final answer:
Joseph is making a 'devil's snare' in the Annunciation Triptych by Robert Campin.
Step-by-step explanation:
In the Annunciation Triptych by Robert Campin, Joseph the carpenter is shown boring a hole into a slab of wood. He is making a mousetrap-like device known as a 'devil's snare,' which is a symbol of protection from the devil interfering with the Annunciation.