Which order correctly arranges machined dental ceramics according to flexural strength from lowest to highest?

The arrangement of machined dental ceramics according to flexural strength from lowest to highest is accurately reflected in the choice provided.

Feldspathic ceramics are typically the weakest in terms of flexural strength due to their composition, which is primarily glassy in nature and lacks the crystalline structure that contributes to greater strength. Following feldspathic ceramics, leucite-reinforced ceramics offer improved strength due to the addition of leucite crystals, which enhance their mechanical properties but still fall short of the performance seen in more crystalline ceramics.

Alumina is a significant advancement in strength as it incorporates a more crystalline structure that enhances its flexural strength even further than leucite. Finally, zirconia is renowned for being one of the strongest dental materials available, primarily due to the transformation toughening mechanism that it exhibits, which allows it to withstand significant stresses without fracturing.

Thus, the correct order starting from the material with the lowest flexural strength (feldspathic) and ending with the ceramic that has the highest (zirconia) gives a clear understanding of the performance capabilities of each material in dental applications.