What characteristic of depleted uranium allows it to self-sharpen?

The correct choice indicates that depleted uranium has a unique characteristic that contributes to its self-sharpening ability. Specifically, this phenomenon is due to the properties inherent in the material's density and its mechanical behavior during impact, particularly in applications like armor-piercing ammunition.

Depleted uranium, primarily composed of U-238, has a high density, which helps it penetrate target materials effectively. However, it does not ignite spontaneously in air; rather, the self-sharpening characteristic refers more to its ability to retain sharp edges upon impact with hard targets, due to its specific structural and physical properties.

The other choices highlight properties that are not directly related to the self-sharpening behavior. While referring to its density does touch upon a factor that aids in penetration, the mention of spontaneous ignition or chemical reactions with water isn't applicable in the context of self-sharpening behavior, as those factors do not directly influence how the uranium interacts mechanically upon impact.

In summary, the self-sharpening capability of depleted uranium is attributed to its density and its metal characteristics, which allow it to maintain sharp edges during use, rather than any reactive nature with environmental factors.