Atomic number of carbon.

**The reasoning** The **atomic number** is the number of protons in an atom's nucleus — and this is what defines an element. Carbon, by definition, is the element with **6 protons**. Therefore, carbon's atomic number is **6**. This never changes. Whether it's carbon in your pencil, in a diamond, or in your body, it always has 6 protons. That's what makes it carbon! **Why the wrong options tempt you** - **A) 4** — You might confuse this with beryllium's atomic number or carbon's common valency (it forms 4 bonds). - **C) 8** — This is oxygen's atomic number. Easy to mix up if you're rushing through the periodic table. - **D) 12** — This is carbon's most common **mass number** (6 protons + 6 neutrons = 12). Many students confuse atomic number with mass number! **Quick takeaway** Atomic number = number of protons = the element's identity; carbon will *always* be 6 because it always has 6 protons. Mass number includes neutrons too, so don't mix them up!