BigFoot is a new online shoe retailer that just hit the market. If a customer chooses to shop at BigFoot rather than Zappos, this is an example of which of Porter's Five Forces?

Threat of new entrants.

Explanation:

Porters Five Forces includes;

The bargaining power of customers, The threat of substitute products or services and others, The bargaining power of suppliers, Competitive rivalry and finally, Threat of new entrants.

However, it is the threat of new entrants scenario we find in Bigfoot's case because Zappos is experiencing reduced market share because of the new entrant (Bigfoot).