1. Hybridization of each carbon in benzene

   Benzene is $\mathrm{C_6H_6}$. Each carbon atom in benzene is $sp^2$ hybridized.

2. Number of $sp^2$ hybrid orbitals formed by one carbon atom

   One $sp^2$ hybridized carbon forms: $3\; sp^2 \text{ hybrid orbitals}$

3. Total number of carbon atoms

   Benzene has $6$ carbon atoms.

4. Total number of $sp^2$ hybrid orbitals in benzene

   Therefore, $6 \times 3 = 18$

5. Check hydrogen atoms

   Hydrogen does not undergo hybridization, so only carbon atoms contribute.

6. Option matching

   $18 \rightarrow \text{Option D}$

Therefore, the number of $sp^2$ hybrid orbitals in one molecule of benzene is 18.