Find the H.C.F of 12 and 24.

Given: 

The numbers are $24 \mathrm{and} 12$.

Factors of  $24 \mathrm{are} 1, 2, 3, 4, 6, 8, 12 \mathrm{and} 24.$

Factors of $12 \mathrm{are} 1, 2, 3, 4, 6 \mathrm{and} 12.$

Common factors of $24 \mathrm{and} 12$ are $1, 2, 3, 4, 6 \mathrm{and} 12.$

The highest common factor of $24 \mathrm{and} 12$ is $12.$

Hence, the correct answer is option A.