"multiple-choice" or "multiple choice"

"multiple-choice" or "multiple choice" is a matter of preference*; "six-letter"or "six letter" is not. In the second case the hyphen would be wrong.

*I prefer the hyphenated form.

So do I.

While both ways of writing the phrase may technically be permitted, the best way is "multiple-choice questions." Hyphens should be used with compound modifiers, and "multiple-choice" is such a modifier here. Using a hyphen makes it clear that the writer intends to refer to questions of the multiple-choice variety, and not a multitude of "choice questions," a reading that is possible when it is written without a hyphen as "multiple choice questions."