Candidly, "forthright" is the right answer. "Frank" and "plain" join "candid" and "forthright" in suggesting a willingness to say what you really feel or think. "Candid" implies sincerity and honesty especially in offering a criticism or an opinion, as in "gave me a candid appraisal of my essay." "Forthright" suggests being direct and clear in expressing your thoughts, as in "made her position clear in a forthright manner." "Frank" stresses lack of shyness or secretiveness and lack of concern for tact, as in "had a frank discussion about improving his grades." "Plain" suggests simple and straightforward expression, as in "ready to hear the plain truth."