In order to find out which of the options would be better to buy we would have to calculate the better unit price that eacho of the following options offer.
So, unit price for the offers would be:
For 4GB flash drive for $8, unit price=$8/4
unit price=$2
For 2 GB for $6, unit price=$6/2=$3
For 8 GB for $13, unit prince=$13/8=$1.625
Therefore, as the unit price of 8 GB for $13 is the lower one, then the better choice to buy would be 8 GB for $13