D — Indian Postal Service
Hindustan Computers Limited, Hindustan Unilever, and Bharti Airtel Limited are all private sector / corporate companies, whereas Indian Postal Service is a government/public sector service organisation.
The key distinction here is private/corporate entity vs. government service. The other three are registered companies (corporations), while India Post is a department under the Government of India. Note: the source passages do not directly address this question, so the answer relies on general knowledge of these organisations. In an MCQ, identify the category each option belongs to and find the one that doesn't fit.