Please go to the store and pick up a _______ of milk ?

In the context of the sentence "Please go to the store and pick up a _______ of milk," the goal is to identify a term that appropriately describes a commonly accepted quantity of milk. Among the given options, "half gallon" is the best choice for several reasons.

Firstly, "half gallon" specifically denotes a standardized volume for liquid products, particularly milk, which is often sold in various-sized containers such as quarts, half gallons, and gallons. This measurement aligns perfectly with typical grocery shopping practices where milk is frequently purchased in half-gallon containers.

In contrast, the other options are less suitable:
- A "bag" (Option A) is an inappropriate descriptor for a liquid, as bags are typically used for solid items.
- A "dozen" (Option C) generally refers to a quantity of 12, which is not applicable to liquid products. It is primarily used to measure units of countable items like eggs or pastries.
- A "pound" (Option D) refers to weight rather than volume, which can lead to confusion when shopping for liquid products.

Therefore, "half gallon" is the only option that accurately and effectively indicates a recognized unit of liquid measurement for milk, making it the correct choice.