The after sales services are included in the product category of market mix. Option C 
Explanation: 
The set of ideas or actions taken by a business entity or a company to promote its product or brand in the market. The four P’s of marketing mix are namely, 
- Product - The end product that is offered for sale. 
 
- Price – The value set for the finished good for selling. 
 
- Place – Direct or indirect markets. 
 
- Promotion – Public relations, advertising, etc., 
 
From the given, discounts comes under pricing, product image used in advertising comes under promotion and substitute goods belong to a different topic. After-sales services comes under the service branch of the product category.