Answer:
c. Threat of regulation
Step-by-step explanation:
Michael Porter's five forces model states factors for assessing an industry's attractiveness. Following are the five forces as per porter:
- Buyer power: Refers to negotiation power of buyers in a industry
 - Supplier Power: Refers to supplier's power to charge a price for inputs. 
 - Threat of substitutes: Refers to competitors already making homogeneous or similar products.
 - Degree of Rivalry i.e the intensity of competition in an industry
 - Threat of new entrants: Threat of new firms entering the industry and gaining a market share.
 
Thus, Threat of regulation is not considered amongst 5 forces that are used to assess industry attractiveness.