Brooks,
Words or symbols used by a system to extract or convert the plain-vanilla-message.
We can use the word soup as code word, if we want to refer to birds.
Now we use a system that encodes my message from birds to soup and the receiver decodes the message from soup to birds.

Niels

Sergei,
There's nothing wrong with being a critical logical thinker :)
But the context of the test itself, the question at hand and last but not least the supplied possible answers, make the answer to your question "I can" :)

Niels

Ke Xu, the problem states that in a pretend language called Q, there is a word CAB. The language Q has 2 rules: 1) ABC is the base word, and 2) if C immediately follows B, then C can be moved to the front of the code word to generate another word.

Because this is a Data Sufficiency question, you have to decide based on these 2 rules, whether only rule 1 is enough to decide whether CAB is indeed a word, or whether both are required, or whether only rule 2 is enough, or whether either rule 1 or rule 2 is enough.

Because we need both rules to decide that CAB is a valid word in language Q, answer C is the right answer.