Answer:
C) downward sloping and straight.
Step-by-step explanation:
The indifference curve is the curve at which the combination of two goods is shown so that the consumer gets equal satisfaction which makes the consumer different.
The perfect substitutes are those goods which are used in place of another. Like the milk, the producer is different but their objective is the same
In the case of the perfect substitutes, the indifference curve is a straight and downward sloping due to the constant marginal rate of substitution of two goods.