What made a number of the Mayflower settlers unique? How is it improbable that their story has come to define the mythological origins of American culture?

The correct answer to this open question is the following.

What made a number of the Mayflower settlers unique was the fact that they had to agree on a new form of government for the future colony to be established in the Americas. It was known as the Mayflower Compact. This set of rules was agreed upon while Pilgrims were traveling in the Mayflower ship to North America. That agreement served as the first set of rules to apply in the Cape Cod colony, in Massachusetts.  

It is improbable that their story has come to define the mythological origins of American culture in that indeed they celebrated one version of what today is known as Thanksgiving, the most important holiday in the United States. However, historians think that the fest was celebrated just one time in 1621 and was not repeated and did not start a tradition.

a, c, & d are correct:

they wanted lands to settle the extra population.

they wanted new markets for english goods.

they wanted to avoid civil war.

i think it was   tsar alexander ii