The correct answer is: A. a movement up a given indifference curve.
The Hicks substitution effect is the change in the quantity demanded of a good that results from a change in its price, holding real income constant. It is measured by the movement along an indifference curve, which represents all combinations of goods that provide the same level of satisfaction to a consumer.
When the price of a good falls, consumers will substitute that good for other goods, because it is now relatively cheaper. This will lead to a movement up the indifference curve, as consumers consume more of the good whose price has fallen.
Option B is incorrect because it describes the income effect. The income effect is the change in the quantity demanded of a good that results from a change in real income, holding the price of the good constant. It is measured by the movement from one indifference curve to another, as consumers move to a higher indifference curve when their real income increases.
Option C is incorrect because it describes the substitution effect and the income effect combined.
Option D is incorrect because it describes a movement from a higher to a lower indifference curve, which would indicate that the consumer is worse off.