By selecting one setting you can convert eligible numeric free response questions to multiple choice. Please ensure you do this at the beginning of the assignment, before adding any questions. Because some questions do not meet the requirements of conversion (having more than 10 unique answer choices, and having at least a 1% range across the answer choice spread) if they are accidentally added to an assignment they will not display any field to input a solution (offering only a grey blob instead*). If you have a favorite question that is not showing up in a search, chances are this conversion box is checked and the question does not meet the requirements.
Go to the assignment settings, and scroll down until you see 'convert numeric questions to multiple choice'
...