What structural element is located over the fireplace opening to support the masonry above?

The correct choice is the lintel. A lintel is a horizontal support beam that spans above openings such as doors, windows, or in this case, a fireplace. It is crucial for distributing the weight of the structure above the opening and preventing the masonry from collapsing. In a fireplace, the lintel supports the weight of the chimney and the masonry above it, helping to maintain the integrity and stability of the wall or facade surrounding the fireplace.

While headers, beams, and trusses serve structural purposes, they are used in different contexts. Headers are typically used in framing for doors and windows, while beams are more general load-bearing structures that can span larger spaces. Trusses are pre-engineered structures often used in roofs or bridges to support load over larger areas, but they are not applicable in the specific function of supporting the masonry above a fireplace opening. Thus, a lintel is the appropriate term for the structural element functioning in this particular scenario.