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"…

This omits the required 's/' command prefix and the 'g' flag, making it an invalid sed expression.

This reverses the redirection operator (using '<' instead of '>') and omits the 's/' command prefix, producing incorrect syntax.

The substitution pattern is missing its closing '/' delimiter and the 'g' flag, so sed would throw a syntax error and no global replacement would occur.

sed 's/bob/Bob/g' letter > newletter uses the correct sed substitution syntax: the 's/' command prefix, the source pattern 'bob', the replacement 'Bob', and the 'g' (global) flag to replace every occurrence on each line rather than just the first. The shell redirection '>' writes the transformed output to newletter without modifying the original file.

The pattern 's/bob. Bob/' is malformed - it contains a literal dot and a space inside the pattern and has no replacement field after the second delimiter, making it invalid sed syntax.