Option 1 -> Complementary goods have L-shaped (right-angled) indifference curves, not straight lines.
Option 2 -> Perfect substitute goods are represented by straight line indifference curves.
Option 3 -> Joint products refer to production relationships, not consumer preferences or indifference curves.
Option 4 -> By-products are secondary outputs in production, unrelated to indifference curve shapes.
Hence, Option 2: Substitute Goods -> Perfect substitutes have straight line indifference curves because consumers are willing to exchange one good for another at a constant rate. The marginal rate of substitution (MRS) remains constant throughout, creating a linear indifference curve with negative slope. For example, if two brands of bottled water are identical to a consumer, they would be willing to trade them one-for-one, resulting in a straight line indifference curve with slope -1. -> correct