A water meter shall be installed in the __________________________ position.

The correct answer indicates that a water meter should be installed in a horizontal position. Installing a water meter horizontally is crucial for ensuring accurate measurement of water flow. This position allows the internal mechanisms of the meter, which are designed to register flow effectively when aligned horizontally, to function properly. When installed horizontally, the water meter can accurately capture the volume of water passing through it without the complications that may arise from air pockets or inaccurate readings associated with other orientations.

While a vertical installation might be acceptable in some cases, it is typically less common and may lead to performance issues depending on the design of the meter. A diagonal orientation is generally not advised, as it can create complications in readings and may not meet plumbing code requirements. This adherence to installation standards ensures that the water meter operates efficiently and reliably, aligning with both industry best practices and code compliance.