Select the most appropriate option to fill in the blank. Her _______ manner of…

The blank needs a word describing a manner of speaking that made it hard for people to grasp her true feelings — i.e. a manner that is unclear or open to several interpretations.

“Ambiguous” means open to more than one interpretation, vague or unclear; an ambiguous manner of speaking naturally leaves listeners uncertain of the real meaning, matching the clue “difficult to understand her true feelings.”

The other words point the wrong way: “forthright” (direct, frank), “candid” (honest, open) and “effusive” (gushing, openly emotional) all describe speech that reveals feelings clearly, so they contradict the context. Hence “ambiguous” is the most appropriate choice.