Your distance from the target determines the choke you need. The choke of a shotgun determines shot string only. It has no bearing on shot speed (velocity) or distance (range). That is, the choke does not alter the shotgun’s power—it just controls how tight or spread out the pellets will be at a specific distance.
Cylinder choke is an unconstricted barrel. The shot string spreads quickly.
Improved Cylinder choke has a slight constriction. It allows the shot string to spread fairly quickly.
Modified choke has moderate constriction. The pellets stay together longer, making the shot string denser and more useful at longer ranges. There is also an Improved Modified choke that is slightly tighter than Modified.
Full choke has tight constriction. The shot holds together even longer making this a good choke for shooting at longer range targets due to the more dense pattern.