You have created a really long letter and after you are done you notice that you used the name "Bob" many times but you forgot to capitalize it in many instances. Which command would replace "bob"…

The syntax `/bob/Bob` is incomplete for substitution in `sed`; it would attempt to print lines matching 'bob', not perform a replacement.

The input and output redirection are swapped (`< newletter` when `letter` should be the input), and the command syntax `sed s/bob/Bob/letter` is missing the quotes and global flag.

This command `sed 's/bob/Bob'` lacks the global (`g`) flag, so it would only replace the first occurrence of 'bob' on each line, not all instances.

The `sed` command `s/bob/Bob/g` performs a substitution where `s` initiates the command, `bob` is the pattern to find, `Bob` is the replacement string, and the `g` flag ensures all occurrences on each line are replaced. The output redirection `> newletter` creates a new file with the changes.

The substitution pattern `bob. Bob/` contains a literal dot and space, and is syntactically incorrect without the `s` command and proper delimiters.