In order to find the unit cost of each option, let's divide the cost by the can size.
So we have:
[tex]\begin{gathered} a\text{.} \\ \text{unit cost}=\frac{3.99}{12}=0.3325 \\ \\ b\text{.} \\ \text{unit cost}=\frac{6.29}{4\cdot6}=0.2621 \\ \\ c\text{.} \\ \text{unit cost}=\frac{3.39}{3\cdot3}=0.3767 \end{gathered}[/tex]The lower unit cost is from option b, so this is the best buy.
