Introduction:
In this analogy, we are given that wax is related to grease. We need to find the option that shows a similar relationship between milk and another substance. To find the correct answer, we need to understand the relationship between wax and grease.

Relationship between Wax and Grease:
Wax and grease are both substances that are commonly used for lubrication purposes. They share the characteristic of being viscous and providing a smooth and slippery surface. When applied to a surface, both wax and grease reduce friction and allow for easier movement.

Relationship between Milk and Curd:
The correct option is "d) Curd". Milk and curd are both dairy products and are closely related. Curd is a product obtained by coagulating milk, which involves the conversion of milk proteins into solid curds and liquid whey. Curd is commonly consumed as a food item and is known for its creamy texture and tangy taste.

Explanation:
Wax and grease are related because they both serve as lubricants, reducing friction and allowing for smooth movement. Similarly, milk and curd are related because curd is derived from milk and is a product of the coagulation process.

Incorrect Options:
Let's analyze the other options to understand why they are incorrect:

a) Drink: While milk is indeed a drink, this option does not establish a relationship similar to the one between wax and grease. The analogy requires a relationship based on the similarity of properties or functions.

b) Ghee: Ghee is a type of clarified butter commonly used in cooking. Although ghee is derived from milk, it does not share the same relationship with milk as wax does with grease. Ghee is a product of further processing milk, whereas wax and grease are both used in their original forms for lubrication purposes.

c) Protein: While protein is an essential component of milk and plays a crucial role in curd formation, this option does not establish a direct relationship between milk and protein. The analogy requires a relationship between milk and another substance similar to the one between wax and grease.

Conclusion:
Therefore, the correct answer is option "d) Curd" as it establishes a relationship between milk and curd similar to the relationship between wax and grease. Curd is derived from milk, just as grease is derived from wax, and both curd and grease serve specific purposes related to their original substances.