A shift in the budget line, when prices are constant, is due to.

Change in the income of the consumer.

The correct answer is Option (2) → Change in the income of the consumer.

Budget line consists of all bundles which cost exactly equal to the consumer’s income. It shows all the possible combinations of two goods that a consumer can buy with a given income and fixed prices. The set of available bundles depends on the prices of the two goods and the income of the consumer. 

• When prices are constant, a change in income causes the budget line to shift:

  • Increase in income → Budget line shifts outward (right).

  • Decrease in income → Budget line shifts inward (left).

Changes in demand, preferences, or utility do not directly shift the budget line — they affect consumer choice or satisfaction, not the budget constraint.