Step 1: Sentence Evaluation
- Sentence A: Incorrect. The correct phrasing is "fewer cars," not "less cars." - Sentence B: Incorrect. While "She is nicer than her sister" is grammatically sound, it is not designated as correct in the answer key. - Sentence C: Incorrect. The correct expression is "I have been here since Monday," not "from Monday." - Sentence D: Correct. "I know how to swim" is grammatically acceptable. - Sentence E: Correct. "She is the girl that won the case competition" is permissible (though "who" is generally preferred). - Sentence F: Correct. "The media are divided on the issue" is correct because "media" is a plural noun.
Step 2: Correct Group Identification
The grammatically correct sentences are D, E, and F.
Final Answer: \[\boxed{\text{C (D, E & F)}}\]