Halliday & Matthiessen (2014: 638):
In a WH- sequence the entire clause is usually omitted except for the WH- element itself, or the item that is the response to the WH- element:
I desperately, desperately need them. – What? – The scissors.
What have you read? – [∅: I have read] Lord of the Flies.
Well I prefer Lord of the Flies. – Why [∅: do you prefer Lord of the Flies]? – Because I don’t think I understood Pincher Martin.
The substitute not may appear in a WH- negative:
The kind of approach to reality and to ideas which the book offers us, is it a realistic book? – No, I don’t think so. – Why [∅: do you] not [∅: think so]?
Substitution is less likely in the positive, except in the expressions how so?, why so?.