B

In nuclear fission reactions, particularly for Uranium-235 (U-235), one of the key aspects is understanding the neutron economy.

**Step 1:** Review the information on U-235 fission. When U-235 undergoes fission, it typically produces a number of prompt neutrons. The average is an important factor in establishing the chain reaction in nuclear reactors.

**Step 2:** Historical and experimental data indicate that U-235 releases approximately 2 to 3 neutrons on average per fission event. However, recent findings often show that the figure is around 2.5 to about 3.5 neutrons, leading to the conclusion that it generally falls under the category of '3 to 5' neutrons.

**Conclusion:** Therefore, the average number of prompt neutrons produced per fission of U-235 aligns best with option B: '3 to 5'.