Which of the following is commonly used to cover cakes for a smooth finish?

Fondant is a type of icing that is commonly used to cover cakes due to its smooth, elegant finish. It is a pliable sugar dough that can be rolled out and draped over a cake, providing a sleek exterior that can also be shaped and decorated in various ways. This smooth surface allows for intricate designs and is often favored for wedding and special occasion cakes.

While other frostings like buttercream and cream cheese frosting can also create a nice finish, they usually do not achieve the same level of smoothness due to their texture and composition. Ganache, made from chocolate and cream, can provide a glossy finish but is generally used for drips or as a filling rather than an all-over cover like fondant. Therefore, fondant stands out as the preferred option for achieving a smooth, polished look on cakes.