The passage tells about two scenarios: words in a title that should always be capitalized, and words in a title that should never be capitalized.
Always: main words, first, and last. Never: Articles, prepositions and conjunctions that are less than 5 letters in the middle of the title
Note that the wording of the Never rule is a bit confusing- but I read it to mean that one should not capitalize a conjunction, article, or preposition if it fulfils two conditions: being in the middle of the sentence and being less than five letters.
This is a must be true question, so we need to find an answer choice that must be true based on the always and never.
We cannot prove A: perhaps there is a word in a title that is a preposition that should be capitalized because it is five letters or longer, not because it is the first or last word.
We can prove B. If a word is in the middle and it should be capitalized, we know that it is not an article or conjunction that is shorter than five letters. If a word in the middle of a title is short and an article/conjunction, then it should never be capitalized, so we know if it is in the middle, and capitalized, it cannot be a short article/conjunction.
We cannot prove C. This ignores the idea of first/last words.
We cannot prove D: This ignores the longer than 5 letters criterion.
E relates to all texts, but we only know about titles from the passage.