What does the word "plumb" mean in the context of determining depth?

In the context of determining depth, the word "plumb" specifically refers to the action of measuring the vertical distance downwards into a substance, typically water or soil. This term originates from the use of a plumb bob, which is a device consisting of a weight suspended from a string that helps find a vertical reference point. When one "plumbs" a body of water, for instance, they are gauging how deep that water is.

The other choices do not align with the definition of "plumb." Measuring weight pertains to the force exerted by gravity on an object, while categorizing involves classifying items based on certain traits. Calculating distance is a broader term that does not specifically indicate vertical measurement, further distinguishing it from the focused meaning of "plumb."