Final answer:
The horizontal member at the bottom of a window opening that supports the window is called the sill. This is distinct from the sash, jamb, or mullion which serve other functions in the window frame's structure.
Step-by-step explanation:
The horizontal member placed at the bottom of a window opening designed to support the window is called a sill. Option 2) Sill is the correct answer. The window sill is the flat piece at the bottom of the window frame. It is an important structural element that holds the window in place and provides a foundation for the window to rest upon. It differs from other parts of the window structure such as the sash, which is the part that holds the glass and can move up and down, the jamb, which is the vertical portion of the window frame, and the mullion, which is a structural element that separates adjacent window units.