I didnt see an explanation for this, so I'll give it a shot.
This is a must be true question with a lengthy stimulus. There is a lot going on here, so we must read through the stimulus with an open mind, knowing that the best way to answer this question is through POE (process of elimination).
(A) is incorrect because the stimulus states "even a written constitution becomes a liberal constitution only when it is interpreted and applied in a liberal way", which shows us how to find a liberal constitution, not a "not liberalone". This could be true, but it doesn't have to be true.
(C) is incorrect because the stimulus never discusses advantages; this answer choice is out of scope.
(D) is incorrect because we are not told in the stimulus about the liklihood on whether a constitution will be liberal or not. Therefore, this answer choice goes way beyond our scope.
(E) is incorrect because this totally misinterprets the last sentence of the stimulus, which states that a "written constitution becomes a liberal constitution only when it is interpreted and applied in a liberal way", which shows that this is a necessary condition (to interpret and apply it in a liberal way) and this answer choice takes it to show that if it is possible to interpret it that way, then it IS that way, which is completely off.
This leaves us with answer choice (B). The reason this answer must be true is because if you noticed the necessary condition of a constitution being liberal when ("when" introduced the necessary condition here) it is interpreted and applied in a liberal way. What I'm trying to say is, it must be true that it is impossible to determine that a written constitution is liberal merely through careful analysis of the written text, since you must ALSO apply it in this way. Therefore, this answer choice must be true.