説く
とく · toku · JLPT N3 · verb-godan
to explain
📢 toku sounds like talk-u: you talk someone through an idea
The left part 言 means speech, so 説く is about using words. It's the same 説 as in 説明 (setsumei, explanation). To explain is to talk it through, and to advocate is to talk for a cause.