Final answer:
The endoplasmic reticulum (ER) performs protein modification in the rough ER and lipid synthesis in the smooth ER. DNA storage is not a function of the ER, and ATP production mainly takes place in the mitochondria.
Step-by-step explanation:
The endoplasmic reticulum (ER) performs several crucial functions in the cell. Out of the options given, the two functions that are indeed carried out by the ER are B. Protein modification and C. Lipid synthesis. Protein modification occurs in the rough ER, where ribosomes attached to the ER make proteins, which may be altered before being transported to their destinations. Lipid synthesis takes place in the smooth ER, which is responsible for the production of various lipids needed by the cell. DNA storage is not a function of the ER, as DNA is primarily stored in the cell's nucleus. Additionally, ATP production predominantly occurs in the mitochondria, not the ER.