Separation of the isotopes of uranium requires a physical ra
     Separation of the isotopes of uranium requires a physical, rather than chemical, method because:  mixing other chemicals with uranium is too dangerous  the isotopes are chemically the same  the isotopes have exactly the same number of neutrons per nucleus  natural uranium contains only 0.7%  U  uranium is the heaviest element in nature 
  
  Solution
Different chemical elements can be purified through chemical processes but isotopes of the same element have nearly identical chemical properties, which makes this type of separation impractical.
So the answer will be oprion (b)

