The correct answer is option 2 - lipid synthesis.

Smooth Endoplasmic Reticulum (SER) is a network of membrane-bound tubules found in eukaryotic cells. It lacks ribosomes on its surface, which gives it a "smooth" appearance. While the primary function of SER is involved in the metabolism and detoxification of drugs and toxic substances, it also has additional roles.

Option 1 - Protein synthesis: This function is primarily carried out by the Rough Endoplasmic Reticulum (RER), which has ribosomes on its surface. The ribosomes on RER are responsible for synthesizing proteins, whereas SER lacks ribosomes, making it less involved in protein synthesis.

Option 3 - Storage of biomolecules: While the SER is involved in the storage and release of calcium ions, which are important for cell signaling, it does not primarily function in the storage of other biomolecules.

Option 4 - Detoxification of toxic substances: This is one of the key functions of SER. It contains enzymes, such as cytochrome P450, which participate in the metabolism and detoxification of drugs, alcohol, and other toxic substances.

Therefore, option 2 - lipid synthesis, is an additional function of the Smooth Endoplasm