Which of the following shell redirections will write standard output and standard error output to a file named filename?

`2>&1 >filename` is incorrect because `2>&1` redirects stderr to where stdout currently points (the terminal), and *then* `>filename` redirects only stdout to 'filename', leaving stderr going to the terminal.

The redirection `> filename` first redirects standard output (file descriptor 1) to 'filename'. Then, `2>&1` redirects standard error (file descriptor 2) to the same destination that standard output is currently pointing to, which is now 'filename'. This ensures both streams go to the same file.

`1>&2>filename` is incorrect syntax; `1>&2` would redirect stdout to stderr, and then `>filename` would attempt to redirect stdout (which is now linked to stderr) to the file, but the original stderr would still go to the terminal.

`>>filename` redirects only standard output to 'filename' in append mode, it does not redirect standard error.

`1&2>filename` is incorrect syntax and does not achieve the desired redirection for both streams.