Why does franklin believe that any constitution the convention approves will be an imperfect document

because humans are not perfect and have their own prejudices and bias so no document will ever be "perfect"

The correct answer because humans are not perfect and have their own prejudices and bias so no document will ever be "perfect."

Explanation:

Benjamin Franklin was an American polymath. He is also one of the Founding Fathers of the United States of America; he was a writer, political philosopher, politician, among many other things. In spite of his concerns, he was the only Founding Father who signed all four of the key documents, the Constitution among them.

the third paragraph is your answer,

"but, in a larger sense, we can not dedicate — we can not consecrate — we can not hallow — this ground. the brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. the world will little note, nor long remember what we say here, but it can never forget what they did here. it is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. it is rather for us to be here dedicated to the great task remaining before us— that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under god, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth. "