Answer : Option 1) nuclei of 
and nuclei of 
only.
Explanation : Radiation is spontaneously emitted from nuclei of 
because this isotope of hydrogen is highly radioactive as compared to other isotopes of hydrogen namely; nuclei of and nuclei of .
They have much stable nucleus as compared to nuclei of .
The more it is unstable the more radiations will be emitted from its nucleus.
The formula of Density is known as:
Density = mass/volume
Units: g/cm^3
Hope this helps!
