The number of neutrons released during the fission reaction of uranium-235 is typically 4. This is a known value for the fission of uranium isotopes, where a single fission event of uranium-235 generally produces 2 to 3 neutrons, but on average, it is 4 neutrons that are released. These neutrons can go on to initiate further fission reactions, leading to a chain reaction. This process is a fundamental aspect of nuclear reactors and nuclear weapons.
Hence, the correct answer is (c).