Words have an opposite meaning of another word are known as antonyms or "antónimos" in Spanish. In the case fo the word "difícil" that is equivalent to the word "difficult" in English, this is an adjective used to describe tasks, activities that require a great effort. For example "El examen es difícil" (The exam is difficult). In this context, the word with an opposite meaning or "antónimo" is "fácil" because this word describes activities that require little effort. Also, other options are not possible as "aburrido" means boring, "interesante" means interesting and "ocupado" means busy which is not related to the word "difícil".
