The correct answer is B. Before
Step-by-step explanation:
A conjunction is a word or set of words used in writing to link grammatical elements, especially to join clauses in a sentence. Due to this, each conjunction is used in particular cases and it expresses a specific relationship such as cause-effect, time, order, condition, etc. In the case of time, conjunctions that express a time relationship include after, then, until, as soon as and before. In the case of before this expresses a time relationship as it shows to the reader one event, action or element precedes another. Thus, the conjunction that expresses a time relationship is before.