Order these units from smallest to largest: one quart, one gallon, one cup.

One cup, one quart, one gallon.

To order these units from smallest to largest, here are the steps:

Step 1: Compare cup and quart.
- One cup is smaller than one quart because there are four cups in one quart.

Step 2: Compare cup and gallon.
- One gallon is larger than one cup because there are 16 cups in one gallon.

Step 3: Compare quart and gallon.
- One gallon is larger than one quart because there are four quarts in one gallon.

So, from smallest to largest:
- One cup
- One quart
- One gallon

To order these units from smallest to largest, you need to compare their sizes. Here's how you can do it:

1. Convert one quart and one gallon to cups:
- One gallon equals 4 quarts.
- Therefore, one quart equals 1/4 (or 0.25) gallons.
- So, one quart is smaller than one gallon.

2. Now, compare one cup to one quart:
- Since one cup is smaller than one quart, and one quart is smaller than one gallon, we can conclude that one cup is the smallest unit.
- Therefore, the correct order from smallest to largest is: one cup, one quart, one gallon.

So, the correct order is: one cup, one quart, one gallon.