An equation that shows how two units of measurement are related is called a

conversion factor.

Explanation:

1) When you see or report a measurement you need to give a number along with its units.

For example, in some one asks how long is a piece of wood and you tell 3, the other person does not know for sure whether it is 3 cm, 3 inches, 3 feet, 3 meters, or even 3 km or 3 miles.

2) As there are many units used for the same magnitudes (as in the example given above) it is necessary to know the equivalence between different units.

That equivalence in the form of equation permits to convert (pass) from one unit to other) and so is a conversion factor.

This are some examples of conversion factor:

1 inch = 2.54 cm ⇒ you multiply a measure in inches by 2.54 to obtain the equivalent measure in cm.

1 kg = 2.2046 pounds ⇒ you multiply a measure in kg by 2.2046 to obtain the equivalent measure in pounds.

1 yard = 3 feet ⇒ you multiply a meassure in yards by 3 to obtain the equivalent measure in feet.

the answer is definitely c.