A. Close to a kilometer, but more often over 2 miles deep.
Why so deep?
The major reason for storing CO2 so deep is that it's required by law. The US Safe Drinking Water Act orders that underground injection is below and isolated from freshwater aquifers. So, carbon dioxide is stored much deeper than most aquifers.
Second, carbon dioxide is most efficiently stored underground in a dense phase (sometimes referred to as supercritical carbon dioxide or scCO2 by chemists). In this dense phase, carbon dioxide has properties of both a liquid and a gas: it is easier to inject into the reservoir than a liquid and once in the reservoir it takes up less space than a gas. This dense phase of carbon dioxide occurs at pressures above 1070 psi (or 73 atm).
Conditions deep underground are very different from the conditions on Earth's surface. The pressures are much higher, just what's needed to keep CO2 in its dense phase. If you could go swimming one mile deep in the ocean, the weight of the water above you would squash the air in your lungs. This is called hydrostatic pressure. Underground, rock formations are saturated with water. The increased weight of water pushing down from above increases the hydrostatic pressure by 43.3 psi with each 100 feet. By the time you reach 2200 feet below the water table (2/3 of a kilometer or 0.4 miles deep), there is so much hydrostatic pressure that carbon dioxide remains in its dense phase.
In Texas and along the Gulf of Mexico, the reservoirs with the best geological characteristics for sequestering carbon dioxide are found even deeper, at depths of about 3.2 km, or 2 miles.