Answer:
Lucas' s best defence would be :
b. He did not know about the trap door.
Step-by-step explanation:
If Lucas didn't know about the trap door,there was no way he could have warned his friends or he would have put any sign that will prevent them from walking through the trap hence preventing them from getting injured.
He could say he would have definitely warned them if he knew but just like them he also didn't know which means he also could have gotten hurt by this trap; I believe this could be the only defence that Lucas can give to try and convince the judge to see his side of the story in this issue.