Answer: A. straight lines.
Step-by-step explanation:
When goods are perfect substitutes, it means that the consumer is willing to substitute one good for another at a constant rate. In other words, the consumer considers the goods to be equally desirable and is indifferent between them. In this case, the indifference curves will be straight lines, indicating that the consumer is willing to give up a fixed amount of one good in exchange for a fixed amount of the other good while remaining equally satisfied.