So to find out how much each bottle costs you have to multiply the cost with the number of bottles.
$3.79 / 6 = $0.63
$4.50 / 9 = $0.5
$6.89 / 12 = $0.57
So the package that cost least per bottle would be the 9 pack for $4.50 because each of the bottles cost $0.5.
Hope this Helps!!