This organelle is involved in lipid metabolism

The correct option is SER.

To understand why this is the correct answer, let us break down the functions of the organelles listed in the options:
1. SER (Smooth Endoplasmic Reticulum): This organelle lacks ribosomes on its surface, giving it a "smooth" appearance. Its primary functions include the synthesis of lipids (such as phospholipids and cholesterol), steroid hormones, and the metabolism of carbohydrates and lipids. It also plays a crucial role in detoxification processes within the cell.
2. RER (Rough Endoplasmic Reticulum): Unlike the SER, the RER is studded with ribosomes. Its main function is the synthesis, folding, and transport of proteins destined for membranes or secretion.
3. Golgi apparatus: This organelle is responsible for modifying, sorting, and packaging proteins and lipids that are synthesized in the endoplasmic reticulum, preparing them for transport to their eventual destinations.
4. Peroxisomes: These are membrane-bound organelles involved in catabolic reactions, particularly the beta-oxidation of very-long-chain fatty acids and the detoxification of hydrogen peroxide, rather than the primary synthesis and anabolic metabolism of lipids.

Therefore, the Smooth Endoplasmic Reticulum (SER) is the organelle directly involved in lipid metabolism and synthesis.