An adverb is a word or set of words that modify verbs, adjectives, or other adverbs. They answer questions such as how, when, where, why, how often, how much etc.
In the sentence given, it says 'we are standing ___ his house waiting for him'.
The question we need to ask is 'Where are they standing?'
The correct word from the options given to answer this question is option A) outside which means situated beyond the boundaries of someplace.
Option B) down is both a preposition (when it follows a noun) and an adverb (when it doesn't follow a noun). In the given sentence, 'down his house' is incorrect grammatically and so option B) is incorrect.
Option C) about is used as an adverb to indicate movement in an area and does not answer 'where they are standing' and so is incorrect.
Option D) Away is used to indicate a distant location from a particular place or person. Generally, it is followed by 'from'. In the given sentence, 'away' is incorrect as the sentence will be incomplete without 'from' following 'away'.
Therefore, the correct answer is option A) outside.