The correct answer is #!/bin/bash. The #! sequence at the beginning of a script is called a shebang or interpreter directive. It instructs the operating system's program loader to use the specified program (in this case, /bin/bash) to interpret and execute the script's contents. This ensures that the script is run with the correct shell, Bash, making it independent of the user's default shell environment.

• SHELL=/bin/bash is incorrect because it only sets the SHELL environment variable within the script's execution context; it does not tell the system which interpreter to use to run the script itself.
• // exec /bin/bash is incorrect. In shell scripting, // does not create a comment; # does. Even if it were # exec /bin/bash, it would be treated as a comment and ignored by the interpreter. The exec command itself replaces the current process, which is not the function of an interpreter directive.
• # This script requires /bin/bash is incorrect because it is only a comment. While good for human readers, a comment line is ignored by the program loader and does not affect which interpreter is used.