The cellular organelle responsible for processing and packaging proteins destined for secretion is the Golgi apparatus.

Here's why:
- Proteins synthesized by ribosomes on the rough endoplasmic reticulum (RER) are first folded and undergo initial modifications within the RER.
- These proteins are then transported in vesicles to the Golgi apparatus, which acts like the cell’s "post office."
- The Golgi apparatus further modifies, sorts, and packages proteins into secretory vesicles.
- These vesicles then transport the processed proteins to the cell membrane for exocytosis, allowing the proteins to be secreted outside the cell.
- The mitochondrion is primarily responsible for cellular energy (ATP) production.
- The lysosome contains hydrolytic enzymes for intracellular digestion but is not involved in packaging secretory proteins.

Therefore, the key function of the Golgi apparatus in the secretory pathway makes it the correct answer.

Reference: Guyton and Hall Textbook of Medical Physiology, 13th Edition, Chapter 2, Page 28