The correct answer is A. gallon.
A yard is a unit of length in the imperial and US customary systems of measurement. It is currently defined as 0.9144 meters. An inch is a unit of length in the imperial and US customary systems of measurement. It is currently defined as 0.0254 meters. A gallon is a unit of volume in the imperial and US customary systems of measurement. It is currently defined as 3.785411784 liters.
A yard is to an inch as a gallon is to a quart. This means that a yard is to an inch as a gallon is to a quart in terms of their relative sizes. A yard is 36 inches long, and a gallon is 4 quarts. This means that a yard is 36 times as long as an inch, and a gallon is 4 times as large as a quart.
The other options are incorrect because they do not have the same relationship to each other as a yard and an inch, and a gallon and a quart. A gallon is not to an ounce as a yard is to an inch. A gallon is not to milk as a yard is to an inch. A gallon is not to liquid as a yard is to an inch.