Dropsy is a disease that is caused by adulteration, specifically adulteration in mustard oil. Mustard oil is a commonly used cooking oil in many regions. Adulteration in mustard oil refers to the act of adding impurities or other substances to the oil, which can be harmful to health.

It is important to note that dropsy is not caused by adulteration in ghee, arhar dal, or turmeric powder. Ghee is a clarified butter commonly used in Indian cuisine and is not typically associated with the adulteration that causes dropsy. Arhar dal, also known as pigeon pea, is a type of lentil and is unrelated to the cause of dropsy. Similarly, turmeric powder, typically used as a spice and natural food colorant, is not responsible for dropsy.

Therefore, the correct answer is option 3, mustard oil. Adulterated mustard oil can contain harmful substances that can lead to dropsy and other health issues. It is important to be cautious and buy mustard oil from trusted sources to avoid adulteration and its potential consequences.