Which part of the cell is responsible for maintaining cell shape and structure?

The cytoskeleton is the correct choice because it acts as a network of protein fibers within the cell, providing structural support and shape. It is composed of three main types of fibers: microfilaments, intermediate filaments, and microtubules, each playing a role in maintaining the cell's integrity. The cytoskeleton also facilitates cellular movement and helps organize the internal components of the cell, making it essential for both shape and function.

The cell membrane does contribute to overall structural integrity by enclosing the cell and interacting with the cytoskeleton, but it primarily serves as a barrier and is not responsible for the internal structural framework. The nucleus plays a vital role in storing genetic information and regulating gene expression, while the mitochondria are mainly involved in energy production through cellular respiration. Neither the nucleus nor the mitochondria contribute directly to maintaining the overall shape and structural integrity of the cell as effectively as the cytoskeleton does.