Final answer:
The diameter of an abdominal aortic aneurysm is best measured using the transverse plane, as it gives a clear cross-sectional view of the structure. The correct answer is A.
Step-by-step explanation:
To measure the diameter of an abdominal aortic aneurysm (AAA), the correct plane to use is the transverse plane. This is because the transverse plane divides the body or organ horizontally into upper and lower portions, providing the best view for measuring diameters across structures. A transverse plane generates cross-sectional images that are particularly useful in assessing the size and extent of an AAA.