Answer:
The correct answer is option d.
Explanation:
The most effective model to understand the effect of change of a variable on other variable is by assuming other factors to be constant. This simplifies the model and helps in easily understanding the relationship between the two variables.
Though the assumption of other things being constant does not apply in the real world, it is still used as otherwise change in other factors would complicate the model. If several factors change it would be difficult to understand the relationship between variables.
Here, to study the effect of change in the price of grapes on the market for wine, it is necessary to assume other factors such as income, consumer preferences, etc to be constant.