The answer is letter b.Β
$4.20 / 60 ounce is the rate for the glass bottle while $3.06 / 51 ounces is the rate for the plastic one.
Simplified, the glass bottle isΒ $0.07 / ounce while the plastic bottle is $0.06 / ounce.Β
Ergo, the plastic bottle is cheaper by a cent per ounce.