The standard input stream, which refers to the keyboard, is called

The correct answer is A. cin.

cin is a standard input stream in C++. It is used to read data from the keyboard. The syntax for using cin is as follows:

cin >> variable;

where variable is the variable that you want to store the data in. For example, the following code will read a number from the keyboard and store it in the variable num:

int num;
cin >> num;

The other options are incorrect. B. cout is a standard output stream. It is used to write data to the screen. C. stin and D. stout are not standard input or output streams.