According to an aspect of the present invention, an aqueous composition contains at least one selected from a group consisting of a carboxylic acid compound represented by the following formula (1), a carboxylic anhydride represented by the following formula (2), and a trimellitic anhydride; a basic compound; and water. In the formula (1), “R” represents an alkanediyl group having 2 to 4 carbons, a vinylene group, or a benzene ring, and “a” represents 2 or 3. In the formula (2), “R” represents an alkanediyl group having 2 to 4 carbons, a vinylene group, or a benzene ring.