According to regulations, how far can an individual's sideburns extend?

The correct choice specifies that sideburns can extend up to 1/8 inch when fully extended. This regulation is in place to maintain a neat and uniform appearance of the Army's personnel, which is a critical aspect of military professionalism. Short sideburns contribute to an overall disciplined appearance that aligns with the high standards required by the Army.

When sideburns are kept at this length, they are less likely to interfere with the proper wearing of headgear or uniforms, and they help maintain a clean, orderly look that is essential in military settings. The fullness and length regulations also aim to present a standardized appearance across all personnel, which is important for unit cohesion and discipline.

In this context, other lengths or measurements suggested would not comply with the Army's guidelines, as they either allow for longer sideburns or do not reflect the established standard, compromising the uniformity required in military appearances.