selecting different categories of values from one column using limit
You could do union on three queries, each selecting only 5 records of choosen complexity :

(select * from question_bank where complexity='high' limit 5) 
union 
(select * from question_bank where complexity='medium'limit 5)


....

This way, you "sum" the records of all your queries (union works only if every request fetch the same columns if I'm not wrong).

You have to be careful to put select statement between parenthesis when using limit and union together (thanks to this answer : https://stackoverflow.com/questions/1415328/combining-union-and-limit-operations-in-mysql-query ), if you fail to do so, your limit won't apply to all select statements.