In the middle course of a river system, certain features are typically observed. These features help to shape the landscape and affect the flow of the river. The options provided are interlocking spurs, ox-bow lakes, river cliffs, and rapids and waterfalls. Among these features, interlocking spurs and river cliffs are commonly found in the middle course of a river system.

Interlocking Spurs:
Interlocking spurs are ridges of land that project into the river valley. They form when the river flows around hard rock outcrops or resistant geological formations. In the middle course of a river, the water has sufficient energy to erode the softer rock, but the harder rock resists erosion. As a result, the river meanders around the interlocking spurs, creating a winding course.

River Cliffs:
River cliffs are steep, vertical slopes that are formed through the process of erosion. In the middle course of a river, the river's energy is relatively high, leading to the erosion of the river banks. As the river erodes the outer bends of meanders, it undercuts the river banks, creating steep cliffs. These river cliffs may be composed of different types of rock, such as sandstone or limestone.

Ox-bow Lakes and Rapids and Waterfalls:
Ox-bow lakes and rapids and waterfalls are not typically found in the middle course of a river system.

Ox-bow lakes are formed when a meander of a river is cut off, creating a U-shaped lake. This process is more common in the lower course of a river when the river has a lower gradient and less energy to erode its banks.

Rapids and waterfalls, on the other hand, occur in the upper course of a river. In this stage, the river has a steep gradient and high energy, resulting in the formation of rapids and waterfalls as the water flows over resistant rock formations.

Therefore, the correct answer is option 'D' - interlocking spurs and river cliffs are the features commonly found in the middle course of a river system.