The correct formula used to determine the formal charge $\left(\mathrm{Q}_{\mathrm{f}}\right)$ on an atom in the given Lewis structure of a molecule or ion is ( $\mathrm{V}=$ number of valence electrons in free atom, $\mathrm{U}=$ number of unshared electrons on the atom, $\mathrm{B}=$ number of bonds around the atom)

• A

  $\mathrm{Q}_{\mathrm{f}}=\mathrm{V}-\left(\frac{\mathrm{U}}{\mathrm{B}}\right)$
• B

  $\mathrm{Q}_{\mathrm{f}}=\mathrm{V}+(\mathrm{U}-\mathrm{B})$
• Correct answer
  C

  $\mathrm{Q}_{\mathrm{f}}=\mathrm{V}-(\mathrm{U}+\mathrm{B})$
• D

  $\mathrm{Q}_{\mathrm{f}}=\mathrm{V}-\left(\frac{\mathrm{B}}{\mathrm{U}}\right)$

Hence, the correct answer is option: 'C'.