Which word means to examine something very carefully?

The word "scrutinize" means to examine something very carefully and thoroughly. This term is often used in contexts where attention to detail is crucial, such as in research, analysis, or inspection processes. Scrutinizing involves not just looking at something superficially, but rather engaging with it critically to uncover specifics, nuances, or flaws that might not be immediately apparent.

In contrast, the other options present different meanings: "retrogress" refers to returning to an earlier and typically worse state, "impetus" denotes a driving force or motivation behind an action, and "quintessence" describes the most perfect or typical example of a quality or class. None of these words connote the careful examination that "scrutinize" does, which reinforces why it is the correct choice.