What is the main ingredient in the restaurant's chowder?

The main ingredient in chowder is typically clams, which is why the correct answer is clams. Chowder, especially the popular New England style, is a creamy soup that is traditionally made with clams, potatoes, and other seasonings. Clams provide a distinct flavor that defines the dish, making them the primary component that distinguishes chowder from other types of soups.

While chicken, beef, and vegetables can be used in various types of soups, they do not represent the classic or most recognized version of chowder, which centers around the use of clams. The essence of chowder comes from the combination of clams and cream or broth, highlighting the importance of clams in this dish.