Which valve controls the flow of gas from the street into a structure?

The curb valve is the correct answer as it is specifically designed to control the flow of gas from the street main into a structure. This valve is typically located underground near the property line and is operated by the utility company. When the curb valve is closed, it effectively stops the flow of natural gas from the supply line to the building, allowing for safe maintenance or emergency response situations.

In context, the appliance cock is used to control the flow of gas to specific appliances within a structure, while the district regulator serves to regulate the pressure of gas supplied to a number of users and does not directly control the flow into individual buildings. The gas meter measures the amount of gas consumed by the structure but does not control the flow, as its primary function is to record usage for billing purposes. Therefore, the curb valve is uniquely responsible for managing the entry of gas into a building from the street.