One____be caerful enough while driving,because a small mistake might cost adriver his life

can‘t
can't be adj.(形容词） enough是固定用法
是再怎么也不为过的意思

can't

one can't be careful enough
没有 太小心 这回事

must

一个人开车时，他必须足够认真。因为一个小的错误可能会使他失去整个生命。

should

表应该

完全正确！

can
仅共参考