Which word does not belong with the others? a. cup b. gallon c. ounce d. liter

The correct answer is c.

Because ounce is a measuring unit of mass. To contrast this:

Cup. It's a US unit of volume. It is common to use this unit with cooking and serving sizes.

Gallon. It's also a unit of volume. This is a unit of measurement for liquid in both the US customary units and the British Imperial.

Ounce. It's also a unit of volume. It's a SI accepted Metric System.