The correct answer is: B. tastes, income and all other prices remain constant.
A demand curve is a graph that shows the relationship between the price of a good and the quantity demanded of that good. The demand curve is downward-sloping, which means that consumers are willing to buy more of a good when the price is lower.
There are a number of factors that can affect the demand for a good, including tastes, income, and the prices of other goods. When tastes change, the demand curve will shift. For example, if consumers suddenly decide that they like a good more, the demand curve will shift to the right. When income changes, the demand curve will also shift. For example, if consumers have more money to spend, they will demand more of most goods. Finally, when the prices of other goods change, the demand curve for a good will also shift. For example, if the price of a substitute good decreases, the demand curve for the good will shift to the left.
In the question, the assumption that tastes, income and all other prices remain constant is important because it allows us to isolate the effect of price on demand. If we were to change any of these other factors, it would be difficult to know whether the change in demand was due to a change in price or a change in one of the other factors.
The other options are not correct because they do not affect the demand for a good in the same way as the assumption that tastes, income and all other prices remain constant. For example, the number of people in a household does not affect the demand for a good. If there are two people in a household, they will still demand the same amount of a good as if there were one person in the household. Similarly, the fact that purchases of a good are made by a free market does not affect the demand for a good. Consumers will still demand the same amount of a good regardless of whether they are buying it in a free market or not.