Which of the following commands replaces each occurrence of 'bob' in the file letter with 'Bob' and writes the result to the file newletter?

`sed '/bob/Bob'` is an invalid `sed` command for substitution; it would attempt to match lines containing 'bob' and then interpret 'Bob' as another command.

`sed s/bob/Bob/` is missing single quotes around the `sed` expression, and `< newletter` would attempt to take input from `newletter`, not write output to it.

`sed 's/bob/Bob'` performs substitution only for the *first* occurrence of 'bob' on each line, not every occurrence as required.

The `sed 's/old/new/g'` command performs a global substitution on each line, replacing all occurrences of 'old' with 'new'. The 'g' flag after the substitution pattern is crucial for replacing every instance of 'bob' on a line, and `> newletter` redirects the standard output of the `sed` command to the specified file 'newletter'.

`sed 's/bob, Bob/'` is an incorrect substitution syntax, using a comma instead of the second `/` and lacking a replacement string after the second delimiter.