Kanji - most of the time on’yomi, but there are exceptions when WK will teach you the kun’yomi for a kanji.
Vocabulary, Solo kanji? kun’yomi. Solo kanji with kana hanging off? kun’yomi. Compound kanji? on’yomi. Compound kanji with kana hanging off? on’yomi.
There are exceptions for every single rule, but that’s gets 80 to 90 percent. There’s a user script that, for kanji reviews, will display what WK expects. I think it’s kind of helpful without being a cheat.