It largely depends on the "voice" of your writing, which is a subjective, stylistic decision. In isolation, either statement reads fine to me, and would be reasonably understood.
If you are already writing in the past tense, I would probably use the past-perfect tense in option B. You might specifically use this form more in a third-person fictional account, to describe a past event that happened to a character that you were already speaking about in the past.
However, autobiographical writing tends to evoke a less-formal style, which might lend itself better to the conciseness of option A.
If this is an exam question, I'm not sure I'd get it right, even as a native speaker (albeit not an academic). I am leaning towards option B, since the surrounding text is already in the past tense, while the choices describe an opinion held before the event of that surrounding text, and that is typically the distinction between how past and past-perfect is used.
The tag past-vs-past-perfect, which was added by an editor, has many, many more examples.