Explanation

Option A is correct because Uranium-235 is the fissile isotope used in nuclear weapons. Option B is incorrect because Uranium-234 is not the primary isotope used for this purpose. Option C is incorrect because while it can be fissile, U-235 is the standard answer for general knowledge questions. Option D is incorrect because Uranium-238 is primarily used as a shield or fuel but is not easily fissionable like U-235.