Leopold I, was Holy Roman Emperor, King of Hungary, Croatia, and Bohemia, he's the second son of which emperor?
Leopold I, Holy Roman Emperor:  The second son of Ferdinand III, Holy Roman Emperor, by his first wife, Maria Anna of Spain, Leopold became heir apparent in 1654 by the death of his elder brother Ferdinand IV. Ferdinand III, Holy Roman Emperor: Ferdinand III (13 July 1608 – 2 April 1657) was Holy Roman Emperor from 15 February 1637 until his death, as well as King of Hungary and Croatia, King of Bohemia and Archduke of Austria.
Ferdinand III, Holy Roman Emperor