The liver is an important organ in detoxication of harmful substances. What organelle in liver cells is active in this process?

The question is correct in stating that the liver plays an important role in detoxification of harmful substances. The liver, being an organ, is made up of liver tissue, and tissue is made of cells. Cells then contain subunits called organelles that perform specific functions inside the cell. The organelle that this question is asking about is the smooth endoplasmic reticulum (smooth ER). It is smooth because it is not studded with ribosomes like the rough endoplasmic reticulum. Both types are present in animal and plant cells; however, cells that specialize in producing lipid and steroid hormones have a greater amount of smooth ER. The smooth ER is also devoted to the metabolism of lipids. In the liver, one of the main functions of the smooth ER is to metabolize and detoxify ethanol (alcohol) and specific drugs like barbiturates.
https://biologywise.com/smooth-endoplasmic-reticulum

https://bscb.org/learning-resources/softcell-e-learning/endoplasmic-reticulum-rough-and-smooth/