Answer:
a. lowest cost per additional unit of utility.
Step-by-step explanation:
A rational consumer tends to maximise utility with regards to per unit cost paid. Minimising 'Cost per additional utility' is also conceptually analogous to it.
Utility maximisation regardless of cost is an incomplete statement, as cost is also a determining factor. Least Marginal Utility per dollar, highest cost per additional unit of dollar : are completely opposite to a rational consumer's choice tendencies.
Example : A consumer consuming two goods (X,Y) get maximum utility where - his/her Marginal Utilities per unit of price paid per good equate . MUx / Px = MUy / Py