___ is high when there are many alternatives to a product or service and low when there are few alternatives from which to choose.

Threat of substitute products or services

Explanation:

Threat of substitute products or services  are one of Porters Five forces.

When there are many substitutes for a product, consumers can easily start consuming a substitute.

For example, there are different brands of cars. If the price of one brand of car, let's say Mercedes increases, consumers can easily start purchasing cheaper brands like Toyota or Avensis .

On the other hand, food is a necessity with almost no substitutes. If the price of food increases, consumers have no other option but to continue buying food as food is needed for survival.