Final answer:
The motivation behind establishing the Constitution as per the Preamble is to secure the blessings of liberty for future generations, indicated as 'securing posterity'.
Step-by-step explanation:
The Preamble to the United States Constitution outlines several motivations for establishing the document. One key motivation mentioned in the Preamble is securing the blessings of liberty to the people of the United States and their posterity. This suggests that the Founders aimed to ensure that the freedoms and rights they valued would be preserved for future generations. Therefore, the correct choice that reflects one motivation behind establishing the Constitution is securing posterity (Option A).