Explanation:
The formula should be independent of the unit in which or for which price and quantities are quoted in order to ensure consistency and accuracy in calculations. This is because different units can be used to express the same quantity or price, and using units that are not compatible with the formula can lead to errors in calculations.

Unit Test:
The unit test is a method of checking the consistency and accuracy of a formula by verifying that it is independent of the units used to express the quantities and prices involved. This test is important because it ensures that the formula can be applied in different contexts and situations without requiring adjustments for different units.

Time Reversal Test:
The time reversal test is a method of checking the consistency and accuracy of a formula by verifying that it remains valid when the time frame is reversed. This test is important because it ensures that the formula can be applied in situations where the flow of time is reversed, such as in financial calculations involving interest rates.

Factor Reversal Test:
The factor reversal test is a method of checking the consistency and accuracy of a formula by verifying that it remains valid when the factors involved are reversed. This test is important because it ensures that the formula can be used in situations where the direction of the relationship between the factors is reversed, such as in elasticity calculations involving price and quantity.

None:
Option 'D' is incorrect because the formula should be independent of the unit in which or for which price and quantities are quoted in order to ensure consistency and accuracy in calculations. Therefore, the unit test is necessary to confirm the formula's validity.