6 pounds bag for $5.40
1) Considering that an eight lbs bag is $7.2, then we can write out the following:

2) So 1 lb of dog food is $0.90. Then examining the options, we can state that the equivalent cost is found at:

Note that we only had to multiply the number of pounds by the unit ratio.
3) Hence, the answer is
6 pound bag for $5.40