A word-usage question asks which word fits, not which word means the right thing.

All four options usually mean the right thing. Meaning is a necessary condition and never a sufficient one. Fit is decided by register, by grammar, and by what the word habitually stands next to — and the exam tests all three.

The minister was reticent about the transfer.

reticent fits a person declining to speak

The minister was taciturn about the transfer.

taciturn describes a permanent disposition, not a topic

Where it bites

Where you can justify two options. That is the signal that meaning has stopped discriminating and one of the other three tests must be applied.