The correct reply, other than a smart-a** response, is to inquire of the questioner if they've had a frontal lobotomy. I thought Huckabee Sanders gave a good response. Because you have to be incredibly stupid to believe any politician would be for slavery in this day and age.
However, if I were the press secretary (and independently wealthy), I would have great fun in first saying, yes, the Trump admin is planning to reintroduce slavery, and then calling the questioner a lot of very bad names while insulting their intelligence.