<span>The argument that Hitler used to justify invading European countries was </span><span>He was rightly getting back the German land taken from Germany in the Treaty of Versailles. This treaty ended World War I.</span>
The correct answer here is the public opinion.
All of the methods mentioned here are ways the politicians may test and see the public opinion. Polls are maybe the main indicator of the public opinion next to voting results but at stage it is a bit late for the candidate. Direct contact with the population is also a method by which the politicians can learn what troubles the said population.