Final answer:
The first step in graft union formation is callus formation, which helps bridge the grafted pieces, leading to the development of new vascular tissue and the connection of their vascular systems.
Step-by-step explanation:
The first step in graft union formation is b) callus formation. During the process of grafting, the cells at the cut surfaces of the plant tissues multiply to form a callus, which eventually allows for the union of the two grafted pieces. This callus is crucial as it forms a bridge that leads to the development of new vascular tissue such as cambium, and subsequently, the connection of the vascular systems (xylem and phloem) between the grafted parts. The formation of the callus precedes the development and differentiation of cambium and the vascular tissues.