Final answer:
Caulk is the material used to seal the joints where the siding meets a horizontal surface over door window frames.
Step-by-step explanation:
The material used to seal the joints where the siding meets a horizontal surface, like over door window frames, is called caulk. Caulk is a waterproof sealant that is applied to fill gaps and prevent water leakage. It is commonly used in construction to seal joints in buildings, including siding joints.