Each of the scenarios is explained as follows:

There are hundreds of colleges that serve millions of students each year. The colleges vary by location, size, and educational quality, which allows students with diverse preferences to find schools that match their needs.

The firms in the given case are colleges. Hence, there are many firms serving the same category of the product but differentiated in nature as per location, size, and educational quality. Entry is challenging because of the regulations in the sector. This is the case of Monopolistic Competition.

There are dozens of pasta producers that sell pasta to hundreds of Italian restaurants nationwide. The restaurant owners buy from the cheapest pasta producer they can. While pasta manufacturers must pay licensing fees to their local government to undergo regular food-safety inspections, anyone who passes inspections can acquire and maintain their license.

There are many pasta sellers. The product is differentiated in terms of pricing and the buyers prefer to purchase the product from the seller who is selling at the lowest price. Entry is challenging because of the local government fee and licensing. Hence, it is a case of Monopolistic Competition.

In a large city, two taxi companies own all the licenses that the city has granted to operate taxis (and the city is reluctant to grant more). Consumers don't care which taxi company they take - if they decide it's worth taking a taxi, they flag down the nearest one.

The number of sellers is few;to be more specific only two. The product is standardized and the consumers show no specific preference towards any one company. The entry is challenging as the market is ruled and divided amongst the existing companies. The is a case of oligopoly rather duopoly with only two suppliers.

Scholastik Inc., owns the U.S. copyright to a popular series of books. It is the only company with the legal right to publish these books in the United States.

There is only one seller. The product is unique and entry to the market is impossible because of the legal copyrights. This is clearly a case of monopoly.