In order for a civilization to become sedentary and to establish towns and cities, -Select- was necessary.

Agriculture was necessary.

Step 1: Understand the question. The question is asking what was necessary for a civilization to become sedentary and establish towns and cities.

Step 2: Consider the history of civilization. Early humans were nomadic, moving from place to place in search of food.

Step 3: Identify the change. The shift from a nomadic lifestyle to a sedentary one occurred when humans began to grow their own food, i.e., when they began to practice agriculture.

Step 4: Formulate the answer. Therefore, for a civilization to become sedentary and to establish towns and cities, agriculture was necessary.