Which of the following structures is part of the cytoskeleton?

The correct structure that is part of the cytoskeleton is the microtubule. Microtubules are one of the key components of the cytoskeleton, alongside microfilaments and intermediate filaments. They are composed of tubulin protein subunits and play crucial roles in maintaining cell shape, enabling cellular movement, and participating in intracellular transport. Microtubules also form the mitotic spindle during cell division, which is essential for the proper segregation of chromosomes.

In contrast, lysosomes are membrane-bound organelles responsible for digestion and waste removal in the cell, while mitochondria are known as the powerhouse of the cell, generating energy through oxidative phosphorylation. The endoplasmic reticulum is involved in protein and lipid synthesis and processing. While all these structures perform vital functions within the cell, they do not contribute to the cytoskeletal framework, which is specifically characterized by the dynamic and structural elements such as microtubules.