Which of the following is not a unit of rigid liquid measure?

The choice of 1 cup as not being a unit of rigid liquid measure is based on the definitions and classifications of volume measurement units. Rigid liquid measures are typically defined by their fixed capacity to hold a certain volume of liquid, usually found in larger standardized units.

While 1 cup is indeed a common unit of measurement in cooking and liquid volumes, it is not usually classified among the more standardized larger units used in industrial or scientific contexts. In contrast, units like 1 gill, 1 1/2 gallons, and 1 liter are recognized standard units of liquid volume that are universally applicable and often used in formal measurement.

The other options represent more sizable and commonly recognized rigid liquid measure units. A gill is a traditional volume measure largely still utilized in some areas, specifically for spirits and beverages. A gallon is a larger unit of volume that is commonly used in the United States and the United Kingdom, whereas a liter is an internationally recognized metric unit of volume. Therefore, the classification of 1 cup as not fitting into the rigid liquid measure category aligns with its more informal use compared to the others.