The correct answer is A. Maintenance of parks and libraries.
Sanitation, public immunisation, and water supply are all mandatory functions of urban local bodies in Jharkhand. Maintenance of parks and libraries is a discretionary function, which means that urban local bodies can choose whether or not to provide this service.
Sanitation is a mandatory function because it is essential for public health and safety. Public immunisation is also a mandatory function because it helps to protect the public from disease. Water supply is a mandatory function because it is essential for human life.
Maintenance of parks and libraries is a discretionary function because it is not essential for public health or safety. However, it can still be a valuable service that provides a place for people to relax and enjoy themselves.