To answer the question about how cells use internal compartmentalization to manage complexity, we need to explore the characteristics of eukaryotic cells and their organelles. Here's a detailed explanation of the options provided:

1. Organelles in eukaryotic cells float freely without any specific role: This statement is incorrect. In eukaryotic cells, organelles are not randomly floating; they have specific roles. For example, mitochondria are responsible for energy production, while the Golgi apparatus is involved in modifying, sorting, and packaging proteins.
2. Prokaryotic cells use multiple membranes to isolate cell activities: This statement is not correct in the context of prokaryotic cells. Prokaryotic cells, like bacteria, generally lack internal membrane-bound organelles, making them less compartmentalized compared to eukaryotic cells.
3. Eukaryotic cells have membrane-bound organelles that separate different functions: This is the correct option. Eukaryotic cells contain membrane-bound organelles such as the nucleus, endoplasmic reticulum, mitochondria, and more. These organelles compartmentalize various cellular processes, allowing eukaryotic cells to manage complexity efficiently.
4. All cellular functions occur in the same space without any separation: This statement is incorrect, particularly for eukaryotic cells. The separation of functions is essential for the efficiency and regulation of cellular activities.

Therefore, the correct answer is the third option: Eukaryotic cells have membrane-bound organelles that separate different functions. This separation is crucial for the regulation and efficiency of various cellular processes, allowing eukaryotic cells to manage complexity effectively.