Assertion (A) The quantity of a commodity demanded invariably changes inversely to changes in its price. Reason (R) The price effect is the net result of the positive substitution effect and negative income effect.

Here is a more detailed explanation of each option:

• Option A: Both (A) and (R) are true. This is the correct answer because both assertion (A) and reason (R) are true.
• Option B: (A) is true, but (R) is false. This is not the correct answer because reason (R) is true. The price effect is the net result of the positive substitution effect and negative income effect.
• Option C: (A) is false, but (R) is true. This is not the correct answer because assertion (A) is true. The law of demand states that, all other things being equal, the quantity demanded of a good or service will decrease as the price of that good or service increases.
• Option D: Both (A) and (R) are false. This is not the correct answer because both assertion (A) and reason (R) are true.