Dear student,

I can understand how much doubt the bookish language have created in your mind even if there are many solution already available for this question but their way is ambigious in itself.

Let me explain in very simple language.

See, when we say 12g gram of carbon it it simply means weight of the carbon sample that we are taking is 12 gram and as you know 12 grams carbon is one mole. And one mole contains 6.022×1023 atoms.

Now let us talk about the most confusing term 12 unit of carbon, see it means that you take 12 atomic mass unit of carbon sample. Here understand that we are not talking about weight of carbon but it is the taking some number of atoms of carbon.

Now 1 Atomic mass unit equals the weight of 1.66 * 10 to power minus 27 kg

So 12 atomic unit of carbon will be 12* 1.66 * 10 to power minus 27 kg.

I hope this simple language clears your doubt.

All the very best!!