Capgemini placement question from previous years, Can any one answer?

Yes, I can answer:

Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons-for example, we may only want a single instance of a pool of database connections.