The correct answer is $\boxed{\text{C}}$. Ethanol is a molecule with the chemical formula $\ce{C2H5OH}$. This means that it has two carbon atoms, six hydrogen atoms, and one oxygen atom. The carbon atoms are bonded to the hydrogen atoms and the oxygen atom, and the hydrogen atoms are bonded to the carbon atoms.
Option A is incorrect because it states that there are four hydrogen atoms in a molecule of ethanol. However, there are actually six hydrogen atoms in a molecule of ethanol.
Option B is incorrect because it states that there are five hydrogen atoms in a molecule of ethanol. However, there are actually six hydrogen atoms in a molecule of ethanol.
Option C is the correct answer because it states that there are six hydrogen atoms in a molecule of ethanol. This is the correct number of hydrogen atoms in a molecule of ethanol.
Option D is incorrect because it states that there are seven hydrogen atoms in a molecule of ethanol. However, there are actually six hydrogen atoms in a molecule of ethanol.