The correct answer is C. Pond.
A pond is a small body of water that is surrounded by land. It is typically shallow and has a slow-moving current. Ponds are home to a variety of plants and animals, including fish, frogs, and turtles.
A river is a larger body of water that flows from one place to another. It is typically deeper and has a faster-moving current than a pond. Rivers are home to a variety of plants and animals, including fish, turtles, and birds.
A lake is a large body of water that is surrounded by land. It is typically deeper than a pond and has a slower-moving current. Lakes are home to a variety of plants and animals, including fish, frogs, and turtles.
A water tank is a man-made structure that is used to store water. It is typically made of concrete or metal and is much smaller than a lake or river. Water tanks are not typically home to many plants or animals.
In conclusion, the simplest aquatic ecosystem can be seen in a pond. This is because ponds are typically shallow, have a slow-moving current, and are home to a variety of plants and animals.