I need help understanding the demand curve and elastic portion:

A city has built a bridge over a river and it decides to charge a toll to everyone who crosses. For one year, the city charges a variety of different tolls and records information on how many drivers cross the bridge. The city thus gathers information about elasticity of demand. If the city wishes to raise as much revenue as possible from the tolls, where will the city decide to charge a toll:

in the inelastic portion of the demand curve, the elastic portion of the demand curve, or the unit elastic portion?