During a wind observation, how is a gust of 21 knots indicated if the 2-minute average wind speed is 17 knots?

In wind observation reporting, gusts are reported alongside the average wind speed to give a more comprehensive picture of the wind conditions. When the average wind speed is 17 knots and there are gusts reaching up to 21 knots, the correct way to indicate this in the reporting format is to focus on the gust value.

In standard aviation reporting, when gusts are mentioned, they are typically expressed in a specific format. The average wind speed is recorded first, and if there is a gust, it is usually indicated as "G" followed by the gust speed. Therefore, the appropriate way to report a gust of 21 knots, alongside an average wind speed of 17 knots, would be to use "17G21" or similar formats that show both average and gust speeds. However, if the choice is only to present the gust value, simply stating the gust speed of 21 knots by itself is considered sufficient.

The other choices might misrepresent the information or include unnecessary context that doesn't directly align with the standard reporting practice, therefore the option indicating just the gust speed as '21' is directly aligned with how gusts can be reported succinctly.