The correct option is B If you ever feel low, you can turn into this journal.
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Here, ‘If you ever feel low’ is an imagined future situation and the future result that is likely to happen is ‘you can turn to this journal’. The sentence means that it’s possible that the person can feel low, and the result is that they can turn to the journal to express their feelings. However, the other option doesn’t talk about an imagined future situation, that’s why it’s incorrect.