Which of the following set of substances contain acids? 

Curd, vinegar

The correct answer is option 4. Curd, vinegar.

In the set "Curd, Vinegar," both curd and vinegar contain acids.

1. Curd (Yogurt): Curd contains lactic acid, which is a mild organic acid produced during the fermentation of milk by bacteria. Lactic acid gives curd its tangy taste.

2. Vinegar: Vinegar is a solution of acetic acid in water. Acetic acid is a weak acid that imparts a sour taste to vinegar. Vinegar is commonly used in cooking and food preparation.

So, in the set "Curd, Vinegar," both substances are examples of foods that contain acids. They are commonly used in various culinary applications, and their acidic properties contribute to their distinctive flavors.