In 1863, President Abraham Lincoln designated the last Thursday in November as a national holiday of Thanksgiving ....... in 1939, President Franklin Roosevelt decreed that the holiday should always be celebrated on the fourth Thursday of the month.
Out of the options given, the answer is "but". This is the right answer as it'll be used to link both sentences regarding the two activities of the President's together.
The other options such as "for example", "therefore" and "so" when used doesn't make the sentence correct.
