The International Zoological club publishes a monthly magazine for zoology enthusiasts. Members are required to pay an initial fee to cover the first two years of the subscription when registering with The International Zoological club. Which competitive advantage is the International Zoological club creating?

locking in customers
differentiating the industry
enhancing the products
establishing alliances

Locking in a customer refers to a situation where customers become dependent on a single supplier for an specific product or service. If they quit on that supplier then they will lose money.

In this case the International Zoological Club makes you pay their product (magazine) two years in advance so if you decide to quit their subscription you´ll lose some money. Therefore so unless you want to give away money, then you are locked in to their magazine for at least two years.