Which parachute canopy has a flat modified design and a diameter of 28 feet?

The parachute canopy that features a flat modified design with a diameter of 28 feet is recognized for its specific applications, often in military operations or precision landing scenarios. The 28-foot canopy provides a balance of stability and load capacity, making it suitable for various missions that require a controlled descent. Its larger diameter allows for greater lift and stability during descent, enhancing its effectiveness when deployed from air platforms.

When considering the other options, it’s clear that parachutes with smaller diameters would not match the specified dimensions and design features required in the question. Canopies such as those measuring 21, 24, or 26 feet lack the necessary size and modified design traits that characterize the 28-foot canopy, thus affirming that the latter is the correct choice.