Number of gallons of water produced each cay by the company = 7200 gallons
Number of 1 gallon bottles put in a carton = 8
Number of cartons needed to hold all the 1 gallon bottles in a day = 7200/8
= 900
So 900 cartons are required to hold all the one gallon bottles produced by the company in a day. I hope the procedure is clear enough for you to understand. It is also important to carefully look at all the details that are provided in the question.
I'd go with the last one because the others don't make sense at all.
I think the helping verb is ''how many''