Assertion (A) : One atomic mass unit is defined as one twelfth of the mass of one carbon-12 atom.

Reason (R) : Carbon-12 isotope is the most abundant isotope of carbon and has been chosen as standard

The correct option is C Both A and R are true but R is not the correct explanation of A
$\text{Analysing Assertion and reason to understand the answer.}$

Analyzing the assertion.
One atomic mass unit is defined as one twelfth of mass of one carbon -12 atom.

$1amu=\frac{1}{12}(MassofC-12atom)$

This statement defines the term atomic mass unit and this is true. Because mass of one carbon-12 atom is 12 amu. So,

$1amu=\frac{1\times 12amu}{12}$


$\text{Analyzing the reason:}$

Carbon - 12 isotopes are the most abundant isotope of carbon and have been chosen as standard.

Carbon has two stable isotopes ${}^{12}Cand{}^{13}C$

• Percentage abundance of ${}^{12}C=98.9\%$
• Percentage abundance of ${}^{13}C=1.1\%$

Thus, it shows $C-12$ isotope is the most abundant isotope of carbon and it is also chosen as standard for reference. So, statement given in reason is also true.

The given Assertion is correct and given reason statement is also correct.

But the reason is not the correct explanation of assertion. It doesn't tell anything about how 1 atomic mass unit is one twelfth of the mass of one $C-12atom.$

Hence, correct answer is option (B).