In Christian church architecture, particularly in cathedrals and larger churches, specific areas are designated for different functions and participants in services.
(a) Transept: The transept is the transverse arm of a cruciform (cross-shaped) church, set at right angles to the nave. It separates the nave from the chancel/choir/sanctuary area.
(b) Nave: The nave is the central part of a church building, intended to accommodate most of the congregation. It extends from the main entrance to the chancel.
(c) Apse: The apse is a semicircular or polygonal termination or recess in a building, especially at the eastern end of a church chancel, often domed or vaulted, where the altar is typically located.
(d) Chancel (and Choir): The chancel is the part of a church near the altar, reserved for the clergy and choir. It is typically located in the eastern part of the church, beyond the nave and transepts. The choir (also called quire) is often specifically the area within the chancel where the choir sits. The sanctuary, containing the main altar, is the most sacred part of the chancel.
The question asks for the section that "houses the clergy and choir during services." This description directly corresponds to the Chancel (which includes the choir area).
\[ \boxed{\text{Chancel}} \]