Which organelle is responsible for processing proteins and other macromolecules for cell secretions?

The Golgi apparatus is the correct answer because it plays a central role in the processing, modification, and packaging of proteins and other macromolecules that are destined for secretion outside the cell or for use within different cellular compartments. It receives proteins synthesized in the endoplasmic reticulum, modifies them by adding carbohydrate or lipid groups, and sorts them to ensure that they reach their appropriate destinations. This organelle acts like a shipping and receiving center within the cell, making it essential for the proper functioning of cellular processes involving secretion.

The nucleus is primarily involved in storing genetic information and controlling cellular activities through gene expression, while the endoplasmic reticulum is mainly responsible for synthesizing proteins and lipids, but it does not perform the final processing and packaging. Mitochondria are known for their role in energy production through cellular respiration, which is unrelated to the processing of proteins for secretion.