Global variables in programming are variables that can be accessed and modified from anywhere within a program. They are not limited to a specific scope or function and can be used by any part of the program.

Global variables are declared outside of any function or class and are typically placed at the top of the program. They are accessible to all functions and can be used to store values that need to be accessed by multiple parts of the program.

Global variables can be classified into two types based on their accessibility:

1. External Global Variables:
- External global variables are declared in one source file and can be accessed by other source files in the program.
- These variables are declared with the 'extern' keyword before their declaration to indicate that they are defined in another file.
- The 'extern' keyword tells the compiler that the variable is defined elsewhere and prevents multiple definitions.
- External global variables allow communication between different parts of a program by sharing data.

2. Internal Global Variables:
- Internal global variables are declared in a single source file and can only be accessed within that file.
- These variables are not declared with the 'extern' keyword and are limited in scope to the file in which they are defined.
- Internal global variables are useful for storing data that is used within a single source file but not needed outside of it.

Therefore, the correct answer is option 'A' - External. Global variables can be external, accessible by multiple source files, or internal, accessible within a single source file.