Answer:
mass of one atom is measured in the unit of 1 amu
Explanation:
As we know that the mass of one atom is very small and it is of order of 
here we know that 1 amu is defined as 1/12 part of mass of 1 atom of C-12 isotope
so here we will have

so here we can say that mass of one atom is measured in the unit of 1 amu
Breaking bond requires energy. The bond between the carbon and hydrogen is broken when the energy is absorbed. The enthalpy is defined to be the energy taken to break the one mole of the stated carbon and hydrogen bond. Thus a should be the correct answer