Choose the idiom/phrase that can substitute the highlighted group of words meaningfully.
The matter is very serious and delicate. So, I urge you to behave or speak carefully to avoid causing problems.

The correct answer is (c) To tread carefully.
Explanation:
The phrase "to tread carefully" means to act or speak cautiously, which fits the context of handling a delicate or serious situation. The other idioms do not fit the meaning required by the sentence.
Meanings of all options:
· In the dark: Unaware or uninformed about something.
· Cut corners: To do something in the easiest or quickest way, often by sacrificing quality.
· To tread carefully: To be cautious about one's actions or words.
· Beat around the bush: To avoid getting to the main point in a conversation.