If a cell lacked smooth endoplasmic reticulum (SER), the most accurate response is:
It would not be able to make lipids.
The smooth endoplasmic reticulum is responsible for the synthesis of lipids, including phospholipids and cholesterol, and also plays a role in the metabolism of carbohydrates and the detoxification of drugs and poisons. Without the SER, the cell would struggle to produce these essential components.