In the context of your analogy, the best match for a "factory" in a city would be the Endoplasmic reticulum.
The endoplasmic reticulum (ER) is involved in the synthesis and processing of proteins and lipids, much like a factory produces goods needed for operation within the city (cell).
If you're looking for other options:
- Golgi apparatus could also be considered, as it further processes, sorts, and ships products made by the ER.
- Ribosomes can be likened to workers in the factory, producing proteins based on instructions from the nucleus.
However, considering your direct analogy of "a factory," the endoplasmic reticulum is the most fitting answer.