Which of the following takes the longest to prepare?

The choice that takes the longest to prepare is a bag of onions. This is due to the fact that onions often require peeling and dicing, which involves specific knife skills to ensure uniformity and to minimize any waste. Additionally, depending on the desired size of the onion pieces and the quantity in each bag, this process can take a significant amount of time.

While tomatoes, potatoes, and bacon also require preparation, they usually involve less intricate work. Tomatoes might simply need to be rinsed or sliced, potatoes can often be cut and cooked with less precision, and bacon is usually precooked or requires less preparation time in comparison to onions, which need careful handling to achieve the right texture and size for cooking. Therefore, the preparation of onions stands out as the most time-consuming task among the options provided.