Among the following transformations, the hybridisation of the central atom remains unchanged in

• A

  ${\mathrm{CO}}_2\to \mathrm{HCOOH}$
• B

  ${\mathrm{BF}}_3\to {\mathrm{BF}}_4^{-}$
• Correct answer
  C

  ${\mathrm{NH}}_3\to {\mathrm{NH}}_4^{+}$
• D

  ${\mathrm{PCl}}_3\to {\mathrm{PCl}}_5$

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