The correct answer is C. Equals the slope of the indifference curve.
An indifference curve is a graph that shows all the combinations of goods that a consumer is indifferent between. The slope of an indifference curve represents the marginal rate of substitution (MRS), which is the rate at which a consumer is willing to give up one good in exchange for another.
A budget line is a graph that shows all the combinations of goods that a consumer can afford given their income and the prices of the goods. The slope of a budget line represents the relative prices of the goods.
At consumer equilibrium, the consumer is maximizing their utility. This means that they are consuming the combination of goods that gives them the highest level of satisfaction. The only way for this to happen is if the slope of the indifference curve is equal to the slope of the budget line.
If the slope of the indifference curve were greater than the slope of the budget line, the consumer could get more satisfaction by consuming more of the good on the vertical axis and less of the good on the horizontal axis. If the slope of the indifference curve were less than the slope of the budget line, the consumer could get more satisfaction by consuming more of the good on the horizontal axis and less of the good on the vertical axis.
Therefore, the only way for the consumer to be at consumer equilibrium is if the slope of the indifference curve is equal to the slope of the budget line.