How is an aircraft at 28,000 feet expressed in flight levels?

The correct response is the expression of altitude in terms of flight levels, which is a standard practice in aviation to maintain clarity and uniformity in communication. When an aircraft is at 28,000 feet, it is referred to as "Flight Level Two Eight Zero." This terminology indicates that the aircraft is operating at a specific pressure altitude, which is standardized for air traffic control purposes.

The reasoning behind using "flight level" is that it helps pilots and air traffic controllers communicate more precisely, regardless of local atmospheric pressure variations that might otherwise affect altitude readings. In contrast, simply stating "28,000 feet altitude," while technically correct, does not follow the standardized phraseology used in aviation.

Using terms like "flight altitude" is not standard aviation terminology and could lead to confusion since the recognized term is "flight level." By stating "Flight Level Twenty-Eight Thousand," you also maintain the proper use of phraseology, but this specific expression aligns with the numeric and phonetic form preferred in aviation communication. This makes "Flight Level Two Eight Zero" the most correct and appropriate choice.