Final answer:
The smallest unit of measure among microliter, megaliter, decaliter, and milliliter is the milliliter (mL), which is equal to a cubic centimeter (cm³) and used for measuring small liquid volumes.
Step-by-step explanation:
The smallest unit of measure among the choices given is the milliliter. A milliliter (mL) is a unit of volume that is equal to a cubic centimeter (cm³). This unit would be appropriate for measuring very small amounts of liquids, similar to the amount of cough syrup one might take. For comparison, a microliter is smaller than a milliliter, but it is also less commonly used. The other units listed, such as megaliter and decaliter, are much larger than a milliliter, and a milliliter is certainly smaller than these.