💡 Detailed Explanation
"～かねない" means "might" or "could," and is used to express the possibility of a negative outcome. It fits perfectly in this context, suggesting that "due to his irresponsible nature, he could very well make a serious mistake." "～に違いない" expresses strong certainty, "～おそれがある" is a more formal warning, and "～わけがない" means "there's no way," which is the opposite of the intended meaning.