Hi Sai chand,
The mistake is in option c because it should be "problems" rather than "problems." This is because the sentence concerns a general scenario of privatization as a response to challenges in the public sector. In such a context, privatization typically addresses multiple issues or shortcomings within the sector as a whole, not just one isolated issue.

Since the phrase is meant to encompass the entire sector and its various challenges, using "problems" is the correct choice, reflecting the broader scope of issues privatization aims to tackle.

I hope this helps.