D. sed 's/bob/Bob/g' letter > newletter Explanation: The sed command is a stream editor used for modifying and transforming text. The s/old/new/g syntax is used in sed to substitute the first occurrence or all occurrences of "old" with "new" in a given input. In this case, the command sed 's/bob/Bob/g' letter > newletter is the correct choice. It reads the contents of the file "letter," replaces each occurrence of "bob" (case-sensitive) with "Bob," and then redirects the output to the file "newletter." The "g" at the end of the command is used as a flag to indicate a global replacement, meaning it replaces all occurrences of "bob" on each line of the file. Option D is the correct command for replacing each occurrence of 'bob' with 'Bob' in the file "letter" and writing the result to the file "newletter."

Correct. student@centos ~]$ cat letter bob bob and john [student@centos ~]$ sed 's/bob/Bob/g' letter > newletter [student@centos ~]$ cat newletter Bob Bob and john