Which word best describes something that is "infinitesimal"?

The term "infinitesimal" refers to something that is so small that it is almost immeasurable. This word is often used in mathematical contexts to describe quantities that approach zero in value but never actually reach it. When considering the context of the word and its implications, "virtually immeasurable" perfectly captures the essence of something being infinitesimal, as it emphasizes the idea of a size or amount that is so small that it cannot easily be quantified or detected.

In contrast, the other options do not align with the meaning of infinitesimal. Descriptions like “harmless and insignificant” and “settled and quiet” encompass different qualities unrelated to size or measurability. “Extremely loud” suggests something entirely opposite in nature, focusing on high volume rather than minuscule scale. Therefore, “virtually immeasurable” is the most fitting choice, as it directly relates to the concept of something being infinitesimally small.