Notice that for the $6.40 package each battery costs:
[tex]\frac{6.40}{8}=0.80.[/tex]For the $15.80 package each battery costs:
[tex]\frac{15.80}{20}=0.79.[/tex]Therefore, the 20 count pack of AA batteries has a lower unit price of $0.79 per battery.
Answer: Last option.