"Before we reach home" is at some undetermined time in the future, so you shouldn't use the present progressive, which refers to the present moment. You must use a future tense. Sometimes we do use the present progressive for the future: I'm studying history next semester.
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Anonymous I wonder why the answer ( a) is not appropriate answer for this blank here.We generally use the present progressive as a way of expressing the future when we are talking of something that has been arranged. The onset of darkness may be certain, but it's not an arrangement.
Englishmaventhe present progressive, which refers to the present moment.It often does not refer to the present moment. It is usually used for a situation of limited duration that happens at or around the present moment: