A type of lot on a U-shaped block is referred to as what type?

A lot on a U-shaped block is referred to as a cul-de-sac. This term specifically denotes a street that ends in a circular turnaround, which can typically be found in residential neighborhoods designed for easier accessibility and to reduce traffic flow. The geometry of a cul-de-sac allows for houses to be positioned at the end of the street, often providing a quieter environment and increased privacy for the residents.

In contrast, the other options refer to different types of lot shapes or configurations. A key lot typically refers to a lot that is partially shaped like a key, often adjacent to a larger parcel. A flag lot is characterized by a long access strip that connects it to the main road, resembling a flag on a pole. A mansard, on the other hand, is not a type of lot but rather a style of roof. Thus, in the context of a U-shaped block, the term cul-de-sac accurately identifies the unique characteristics associated with that specific layout.