Which of the Following Is a Clearly Defined Research Problem

Which of the following is a clearly defined research problem that might be asked by a McDonald's manager?

A) How can we increase sales?
B) Will emailing discount coupons to existing customers lead them to make more purchases in the month after the coupons are emailed out?
C) Should we match our competitors' prices?
D) Should we increase our advertising budget by 15 per cent in the lead-up to the school holidays?
E) Will increasing advertising expenditure work?