Conchita decidió decirle la verdad a María.

when you have clarifying phrases like ‘a Maria’, this usually will be clarifying an indirect object pronoun like ‘le’. So when you use the phrase ‘A (alguien) …’, you have to include the ‘redundant’ pronoun. so it is not ‘decir a maria’, it is ‘decirle a maria’ to clarify it is being told to Maria.