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

The expression '/bob/Bob/' is missing the leading 's' command, making it an invalid sed address range rather than a substitution.

The redirection operator '<' reads from newletter as input instead of writing to it, and the substitution is missing the 'g' flag and uses incorrect syntax.

The substitution expression 's/bob/Bob' is missing the closing delimiter '/', making it syntactically incomplete and causing a sed error.

The command 'sed 's/bob/Bob/g' letter > newletter' uses the correct sed substitution syntax with all three delimiters present, and the 'g' flag ensures every occurrence of 'bob' on each line is replaced. Output is redirected with '>' to write the result to newletter.

The expression 's/bob, Bob/' contains a comma after 'bob' making it match a literal comma, and the replacement field is empty, so nothing meaningful is substituted.