What is the primary function of cytoplasmic streaming in cells?

The correct answer is to transport materials within the cell. 

Cytoplasmic streaming, also known as cyclosis, is the movement of the cytosol and organelles within the cell. This movement facilitates the transport of materials such as proteins, nutrients, and organelles to different parts of the cell. It also helps to distribute materials evenly throughout the cell and aids in the removal of waste products. 

While cytoplasmic streaming may indirectly affect cell metabolism and energy storage, its primary function is to transport materials within the cell. Maintaining the shape of the cell is mainly attributed to the cytoskeleton and the rigidity of the cell wall (in plant cells).