Qu 17. What is backslash in c?

The backslash is a trigger. It says the next character is special. \n is new line, \t is tab, \b is backspace. So logically, the backslash alone can't be used to display a backslash, so you use \\ to display one backslash. 

Backslash Commands 

When used in a character or string expression, the backslash character (\) is used to
give instructions to cout.   For example, \n has the identical effect of endl.      For example:

cout << “This is on one line.” << ‘\n’ << “This is on another line.”;

will produce the output

                         This is on one line.
                         This is on another line

The only advantage of \n is that it can be included in a string.      For example:

                 cout << “This is on one line.\nThis is on another line.”;

will also produce the output

                         This is on one line.
                         This is on another line