Barb, a single mother of five children, chooses to go to Payless Shoes to purchase her children's shoes rather than shopping at Zappos. This is an example of which of Porter's Five Forces?

Rivalry among existing competitors

Explanation:

Porter five forces of the model includes rivalry among competitors, bargaining power of suppliers, bargaining power of buyers, the threat of new entrants, the threat of substitution.

The rivalry among competitors deals with rivals ' strengths and weaknesses so that the company does the strategy accordingly.

The bargaining power of suppliers stated the shift in the price of the product made by the supplier's offer plus the buyer is attracted to the product because the product is exclusive and has an impact on the overall profit

The bargaining power of buyers trades with the number of purchasers and how many orders a single purchaser commits.

The threat of new entrants impacts an overall business position once the competitor comes on the market.

The threat of substitution is an alternative option of producing goods and services that can also have a direct impact on your role and on profits through which the profits got reduced.

So, according to the given statement, the most appropriate option is rivalry among existing competitors