Which word signifies to an excessive extent or degree?

The choice that signifies to an excessive extent or degree is "overly." This term conveys a sense of exceeding what is reasonable or necessary, implying that something is more than just a little excessive. For example, if someone is described as "overly cautious," it suggests they are being excessively careful, going beyond what is typically considered prudent.

Though the option "too" might seem fitting as it is often used to imply excessiveness, it generally serves as a more informally nuanced term relating to something being more than desired or tolerable. The term "very" denotes a high degree but does not inherently communicate excess in the same manner as "overly." "Altogether" typically means completely or entirely and does not pertain to measuring degrees of excessivity at all.

Thus, "overly" effectively captures the concept of being excessive in a contextually appropriate manner.