The correct answer is admitting new states.
Article IV Section III Clause 1 of the US Constitution directly gives Congress the power to add states to the Union besides the 13 that existed at the time when the country started. This expressed power has become an essential part in shaping America today, as 37 states were added due to this clause in the US Constitution. These states would start out as territories, apply for statehood, and then meet several different requirements in order to enter the US as an official state.