I am going to Poland (Delhi-Frankfurt-Warsaw) with Polish National Visa. Do I need Transit visa for layover in Frankfurt?
Since April 2010, holders of national D-type visas of a Schengen country can travel freely in the Schengen area, as long as they observe the 90/180 day rule for the time they spend on other Schengen countries than the one their visa is for.
This means that your Polish D-visa will allow you to enter the Schengen area at Frankfurt and proceed within Schengen to Poland. You don't need an additional visa for that.