Just to add...if you want to ask a question, picture what the interview will say back.
In this case, if you were them, what would you say? Would you reasonably expect them to say anything other than "what do you think?"
Better, think about what you need to do to figure out why complaints increased? What information do you need (suverys, time-series data, information about products, etc.)? What are some possibilities to why (and work backwards from this).
Importantly, what are the possibilities in terms of the context given? You'd be surprised how much information is in the following:
1) Cola - Product issues could be fiziness or taste. Changing tastes are occuring in drinks.
2) New Plant - Quality control issues? Production problems? Delays?
3) London - Demanding demographic group. High expectations. Timeliness super important
4) Inventory doubled - hard to manage this increase (mixed up orders?), how's distribution, etc.