The correct answer is C. String.
A string is a structural element of a staircase that supports the treads and risers. It is typically made of wood, metal, or stone, and is often curved to follow the shape of the staircase. The strings are usually attached to the wall or to a central support beam, and they help to keep the staircase in place.
A baluster is a vertical post that supports the handrail of a staircase. It is typically made of wood, metal, or stone, and is often decoratively carved or turned. The balusters are usually spaced evenly along the length of the staircase, and they help to prevent people from falling off the edge.
A header is a horizontal beam that supports the weight of the ceiling above a door or window opening. It is typically made of wood, metal, or concrete, and is often reinforced with steel rods. The header is usually installed at the top of the opening, and it helps to prevent the ceiling from collapsing.
A beam is a structural element that supports the weight of a structure above it. It is typically made of wood, metal, or concrete, and is often rectangular or I-shaped in cross-section. The beams are usually installed parallel to each other, and they help to keep the structure from collapsing.