Which preposition should I use here:You are not listening! What are you thinking __ ?
can i use thinking for?
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There are three valid answers that I can think of (see what I did there?)
What are you thinking? [ie, no preposition needed at all]
What are you thinking of?
What are you thinking about?
If you said "What are you thinking for?" it would mean the same as 'Why are you thinking?/For what reason are you thinking?', which does not make sense in the context of the previous sentence.