Explanation:
The correct answer is option 'b' - "In."

In this sentence, the word "deals" indicates that the shopkeeper is involved in the trade or business of cosmetics. The preposition "in" is used to show involvement or participation in a particular activity or business.

Example:
The shopkeeper deals in cosmetics.

Here, "in" shows that the shopkeeper is engaged in the business of selling cosmetics. It indicates that the shopkeeper specializes in or is involved in the trade of cosmetics.

Other options:
- Option 'a': "With" is not the most appropriate option in this context. The preposition "with" is used to indicate possession or association, but it does not convey the meaning of being involved in a particular business or trade.

Example:
The shopkeeper deals with various products.

Here, "with" is used to show that the shopkeeper is involved in dealing with or handling various products, but it does not specify the specific nature of the products.

- Option 'c': "At" is not the most appropriate option in this context. The preposition "at" is used to indicate location or position, but it does not convey the meaning of being involved in a particular business or trade.

Example:
The shopkeeper deals at the local market.

Here, "at" is used to show the location or position where the shopkeeper conducts their business, but it does not specify the specific nature of the business.

- Option 'd': "For" is not the most appropriate option in this context. The preposition "for" is used to indicate purpose or recipient, but it does not convey the meaning of being involved in a particular business or trade.

Example:
The shopkeeper deals in cosmetics for beauty enthusiasts.

Here, "for" is used to show the purpose or the target audience for whom the shopkeeper sells cosmetics, but it does not specify the involvement in the business itself.