What is one characteristic of nanoparticles?

Nanoparticles are defined by their size, typically in the range of 1 to 100 nanometers. One of their key characteristics is that they often display unique physical and chemical properties that differ significantly from those of bulk materials due to their small size and high surface area-to-volume ratio. Option C is correct because nanoparticles represent some of the smallest particles that retain the essential properties of the larger material. Their size facilitates interactions at the molecular and atomic level, which is why they are studied for applications in various fields such as medicine, electronics, and materials science.

In contrast, other options do not accurately describe nanoparticles. While they do contain a considerable number of atoms, the defining feature is not merely the quantity of atoms but rather the size and their resultant properties. They can indeed be formed from larger structures, but this is more about the process of breaking down materials rather than defining their character. Lastly, nanoparticles are typically too small to be observed with the naked eye, further distinguishing them from larger particles which are visible.