Which type of carriers transports goods only under specific contracts?

The correct answer is private carriers because they are businesses or individuals that transport goods for specific clients under contract. This means that the transportation services provided by private carriers are not open to the general public; instead, they agree to transport goods only for certain customers under terms specified in the contract. This exclusivity allows them to tailor their services to meet the unique needs of their clients.

In contrast, common carriers are obligated to serve the public and must offer their services to anyone willing to pay their fee. Public carriers operate similarly to common carriers, providing services widely without the contractual limitations that private carriers have. Freight carriers is a broader term that can encompass both private and common carriers, but it does not specify the contractual nature of the service being offered. Therefore, private carriers are indeed the best choice for transporting goods under specific contracts.