The correct answer is C. fstream.

The fstream class is a C++ class that provides file input and output. It is derived from the iostream class, which provides input and output to the standard streams (stdin, stdout, and stderr). The fstream class provides a number of methods for reading and writing files, including open(), close(), read(), write(), and flush().

The other options are incorrect. The iostream class is used for input and output to the standard streams. The filed class is not a standard C++ class. The diskclass class is not a standard C++ class. None of the above is not a valid option.

Here is an example of how to use the fstream class