The given options are compound adverbs which are contracted to a word with 'a' added to it, for example 'on foot' is contracted to 'afoot'. The speaker says that when parents are away, there's always mischief, and an adverb is required to modify "mischief". From the options, 'afoot' is most suitable, it means 'in progress', like the mischief when the children's parents are away. Hence option D is correct. 'Asleep' is 'a state of sleeping', and mischief can't be asleep, even metaphorically, the sentence is about mischief that takes place, it isn't dormant, or 'asleep'. Hence option A is incorrect. 'Ahead' is 'to advance' or 'in front of', and isn't relevant to the statement. 'Ahead' is a direction and does not modify 'mischief'. Hence option B is incorrect. 'Away' means 'to be gone', and 'mischief' doesn't go, but it's in progress in the sentence, hence option C is also incorrect.