1. Identify the general formula of sulphonic acids

   Sulphonic acids contain the functional group: $-\mathrm{SO_3H}$

   A common example is: $\mathrm{R-SO_3H}$ where $\mathrm{R}$ is an alkyl or aryl group.

2. Check the options

   • A: $-\mathrm{SO_2}$
     This is not the sulphonic acid functional group.

   • B: (blank)
     No valid group given.

   • C: $-\mathrm{SO_3H}$
     This is exactly the functional group of sulphonic acids.

   • D: $-\mathrm{SO_4H}$
     This is not the correct functional group for sulphonic acids.

3. Conclusion

   Therefore, the functional group present in sulphonic acids is: $-\mathrm{SO_3H}$

So, the correct option is C.