What does "QNH" indicate in aviation weather reporting?

In aviation weather reporting, "QNH" specifically indicates the altimeter setting that is adjusted to sea level pressure. This measurement is critical for pilots, as it allows them to calibrate their altimeters correctly, ensuring that they have the most accurate altitude information relative to sea level. When a pilot sets their altimeter to the QNH value provided by weather reports or ATC, they are able to fly at the correct altitude above the ground, which is especially important for safety during takeoff and landing.

Using QNH is vital in aviation as it helps in maintaining appropriate vertical separation between aircraft and avoiding terrain obstructions. An accurate understanding of altimeter settings ensures pilots know their aircraft's altitude in relation to mean sea level, which is essential for safe navigation and landing procedures. This is why B is the correct answer.