Concept:A mortise and tenon joint is the standard joint for joining a rail to a leg at a right angle in furniture frames.Explanation:The leg and front rail meet at a corner where strength and stability are needed.A mortise and tenon joint locks the rail into the leg and resists pulling apart.Dovetail is used for drawer sides, cross halving for crossing pieces, and tongue and groove for edge-to-edge boards.Therefore, the joint at Z is the mortise and tenon joint.Answer:C. mortise and tenon.