Atomic number of carbon.

**The reasoning** The **atomic number** is simply the number of protons in an atom's nucleus. It's what defines the element itself — it's like the element's ID number on the periodic table. Carbon always has **6 protons** in its nucleus. This never changes. If an atom has 6 protons, it's carbon. If it has a different number, it's a different element entirely. So carbon's atomic number = **6**. This is a fundamental definition you need to memorize: Carbon = 6 protons = atomic number 6. **Why the wrong options tempt you** - **Option A (4):** This might confuse you if you're thinking of carbon's valency (4 bonds it can form), but valency ≠ atomic number. - **Option C (8):** You might mix this up with oxygen's atomic number (8). - **Option D (12):** This is carbon-12's **mass number** (protons + neutrons = 6 + 6 = 12). Mass number and atomic number are different! The atomic number is always the smaller number and appears at the bottom left of the element symbol. **Quick takeaway** Atomic number = number of protons = the element's permanent identity; for carbon, it's always 6.