Explanation:
The correct answer is C: mitochondria.

The mitochondria are the parts of the cell solely responsible for respiration, particularly aerobic respiration. During this process, mitochondria convert glucose and oxygen into carbon dioxide, water, and energy in the form of ATP (adenosine triphosphate), which the cell uses for various functions.

Here's why the other options are incorrect:
- A. nucleus: The nucleus contains genetic material and is responsible for controlling cellular activities, including cell division and gene expression, but not for respiration.
- B. nucleolus: The nucleolus is found inside the nucleus and is primarily involved in the production of ribosomes, not respiration.
- D. Golgi apparatus: The Golgi apparatus is involved in modifying, sorting, and packaging proteins and lipids for transport, not respiration.
- E. endoplasmic reticulum: The endoplasmic reticulum (ER) is involved in protein and lipid synthesis and transport, but it does not directly carry out respiration.

Thus, mitochondria are the organelles responsible for respiration in the cell.