At what depth is a trench that measures 6 feet deep?

The depth of a trench is defined as the vertical distance from the top of the trench to the bottom. In this case, if the trench is specified to measure 6 feet deep, that directly indicates the depth itself. Answering with 6 feet is straightforward because it matches the measurement provided in the question.

The other choices present depths that do not align with the provided measurement. A depth of 4 feet would be shallower than specified, while 8 feet and 10 feet would both indicate a trench that is deeper than what is stated. Therefore, the measurement of 6 feet is accurate and represents the true depth of the trench as described.