Concept:"Transpire" means to happen or take place (it can also mean to become known).Solution:"What transpired between them" means "what took place between them".Replace the word: "What happened between them remains a secret" keeps the meaning exactly.A. "manifested" means shown clearly, the opposite of a secret.B. "translated" means changed into another language; it only looks a little like "transpired".C. "existed" means was present or real; it does not describe an event taking place.Key point: Test each option by putting it in place of the underlined word and reading the sentence again.Answer:Option D (happened)