The Stem of a multiple choice item is the part to which the student is to respond. In everyday language, we'd call this the question, but since it could be a statement or even an analogy or an equation, we'll use the technical term, Stem.
The Options are the choices from which examinees are to choose. There are two kinds of options: the Key is the correct or best choice; the Distracters are incorrect or less appropriate choices.
Sometimes Stimulus Materials can also be used with a multiple choice item, something like a bar graph, a table, a map, a short text, etc. There will be more about stimulus materials in a moment.