Answer:
A. shift the budget constraint in a parallel fashion.
Step-by-step explanation:
Budget Line/ Constraint is graphical representation of product combinations that a consumer can buy, given product prices & income (all spent).
Income change will lead to parallel shift in budget constraint, because of equivalent proportional change in purchasing power of both goods.
If Income increases: the consumer can increase consumption of both goods with risen income, because of increased real income/ purchasing power due to price fall. This will shift budget constraint rightwards / outwards.
Similarly: Income decrease would decrease real income/ purchasing power for both the goods, enabling less consumption of both. This will shift budget constraint leftwards / inwards.
Both options B & C are inapt because : Budget Line Rotation occurs in case of price & purchasing power change only in one good or disproportionately in two goods.