The correct answer is A. live.
The verb “live” is the most common verb to use in this sentence. It means to reside in a particular place. In this case, the speaker’s parents live in a very small flat.
The verb “open” is not a good choice because it means to make something accessible or available. In this case, the speaker’s parents are not making their flat accessible or available to anyone.
The verb “speak” is not a good choice because it means to communicate using words. In this case, the speaker’s parents are not communicating using words.
The verb “take” is not a good choice because it means to carry or transport something. In this case, the speaker’s parents are not carrying or transporting their flat.