What would be in out.txt?
cat < in.txt > out.txt
Nothing. The redirect will create a new empty file but there will not be any output from the cat command to redirect.
It would be the contents of in.txt.
Nothing because you can't redirect from file (in.txt) to another file (out.txt). You can only redirect from a command to a file.
The output from the command line. By default STDIN comes from the keyboard.