Which position involves lying fully flat on the back?

The correct choice is indeed Supine. This position is characterized by lying fully flat on the back. It is often used in physical therapy and medical settings since it provides optimal access to the front of the body for examinations, treatments, or procedures. The supine position is also commonly used for patient comfort during a range of therapies and for certain types of imaging studies.

In contrast, Fowler's Position refers to sitting or lying back at an angle, typically between 45 to 60 degrees, which is useful for patients with breathing difficulties or those who require more upper body elevation. The Trendelenburg Position involves lying flat on the back with the legs elevated higher than the head, often used in emergency scenarios to promote venous return to the heart. Lastly, the Prone Position involves lying face down, which serves different therapeutic purposes and is not the correct answer when describing a fully flat position on the back.