Sasha has run a small diner near the train station for the past ten years. Six months ago, a chain restaurant serving gourmet burgers and sandwiches opened up near him and he has had to re-think his menu. He added some specialty sandwiches and free fries with every burger in order to retain his customers. Which of Porter's five competitive forces has affected Sasha's business?

threat of new entrants

Explanation:

Based on the information provided within the question it can be said that force that has affected Sasha's business, from Porters five forces was the threat of new entrants. This force refers to the threat that comes from new competitors entering an industry with existing competitors. If the barrier to entry of the market is low/easy for these new companies then it creates a huge threat to the existing company's since it allows them to get established in the market fast and at a low cost.