At the Constitutional Convention of 1787, the delegates addressed the issue of slavery in several ways, leading to a compromise. Among the options you listed, the correct aspect is:
They decided that the slave trade could not be banned until 1808.
This decision was part of the compromise to gain the support of Southern states for the new Constitution. Additionally, while the Constitution did not explicitly mention slavery, it included clauses that protected the institution of slavery, such as the Three-Fifths Compromise, which counted enslaved individuals as three-fifths of a person for purposes of representation and taxation. Individual states were allowed to regulate the slave trade, but the federal government could not intervene until 1808, at which point Congress could ban the importation of slaves if it chose to do so.
Overall, the Convention did not opt for a complete ban on slavery, nor did they allow for a uniform approach across all states regarding the practice and regulation of slavery; rather, they created a framework that allowed those issues to be largely determined by individual states.