Trade was foundational to the growth of cities because it

(a)guaranteed an exchange of goods and ideas
(b)made reliance on other civilizations possible
(c)showed military strength
(d)gave people jobs

The correct answer is A)guaranteed an exchange of goods and ideas. During most of history, most people were agricultural workers who lived in the countryside, and cities were far smaller than they are now, and were the place where mostly a special kind of people lived: merchants and artisans. The merchants lived in cities because it was there where they could develop a network of clients, and the artisans lived in cities because it was there that they could supply the merchants most efficiently.

This is the same reason why most cities were built close to a water source, be it a river or the sea. This is true for practically every single city that has been succesfull throughout history, some examples are:

Constantinople/Istanbul - which lies on the coast of the strait of bosphorus, that connects the mediterranean sea with the black sea.

Baghdad - which is located along the Tigris River, a major waterway in Mesopotamia, and was the capital of the islamic world for centuries.

New York City - which was founded by the Dutch as New Amsterdam on the southern tip of the island of Manhattan, in what is perhaps the best port in the eastern United States.

And like this, we could mention hundreds in not thousands of cities which were founded near water in order to faciliate the exchange of goods and ideas.

d is the answer. leo should include the source.