What is the distinguishing characteristic of long hair according to Army standards?

Long hair, according to Army standards, must be neatly and inconspicuously fastened. This characteristic ensures that the hair does not interfere with the proper wear of the uniform and aligns with the Army's emphasis on professionalism and military bearing. By requiring that long hair is secured and not left loose, the standards facilitate a clean and orderly appearance, which is crucial in a military environment. This characteristic is vital not only for maintaining a uniform look amongst service members but also for safety and effectiveness during various tasks.

The other options do not align with Army standards. For instance, allowing hair to be worn loose and unkempt, or leaving it hanging freely, contradicts the uniformity and discipline the Army upholds. The regulation regarding hair bulk is also more stringent, focusing on specific measurements that must be adhered to, which are not represented in the correct answer. Overall, the requirement for hair to be neatly and inconspicuously fastened embodies the Army's values of discipline and professionalism in uniform appearance.