Final answer:
Thomas Jefferson was not present at the Constitutional Convention as he was the American ambassador to France at that time. Therefore, the correct option is A.
Step-by-step explanation:
The individual who was not present at the Constitutional Convention was Thomas Jefferson. Jefferson was serving as the American ambassador to France during the time the convention took place. While figures like George Washington, Benjamin Franklin, and James Madison played significant roles at the convention, Jefferson was abroad and could not attend, although he kept abreast of the proceedings from afar.