Which word means to examine thoroughly and in great depth?

The word "plumb" is correctly chosen as it means to examine something thoroughly and in great depth. This term often implies a meticulous investigation, akin to measuring the depth of water with a weight on a line, which evokes the sense of getting to the bottom of a subject or issue. In a figurative sense, to "plumb" a topic is to investigate it deeply, uncovering its complexities and nuances.

While "probe" and "explore" can involve thorough examination, they do not necessarily carry the same depth connotation as "plumb." "Probe" suggests a more surface-level investigation and can imply invasive questioning, while "explore" generally denotes a search or journey without the same focus on depth. "Dive," on the other hand, often implies a quick or direct entry into a subject without necessarily suggesting a comprehensive examination. Therefore, "plumb" stands out as the term that embodies a meticulous and in-depth examination.