The most appropriate meaning of the idiom "to be in the doldrums" is to be in low spirits. The idiom comes from the nautical term "doldrums," which refers to a belt of calms and light winds near the equator where sailing ships were often becalmed. In this sense, the doldrums are a metaphor for a state of inactivity or stagnation. When someone is "in the doldrums," they are feeling down, bored, and unmotivated. They may lack energy and enthusiasm, and they may have difficulty concentrating or making decisions.

The other options are not as accurate descriptions of the meaning of the idiom. Feeling anxious is not the same as being in low spirits, and living in a bunker is not a common expression for being in a state of inactivity. Thinking deeply over something is not typically associated with the doldrums, as it is often a sign of focus and productivity.