Final answer:
Denny was aiming his gun at Bonnie's right breast, which Venters later discovers after shooting her.
Step-by-step explanation:
The body part that Denny was aiming his gun at Bonnie was her right breast.
This is revealed in the passage 'Venters knelt with a gathering horror of his deed. His bullet had entered the rider's right breast, high up to the shoulder.' The passage also emphasizes Venters' realization that he had killed a girl.
This excerpt is from a book or story that falls under the subject of Social Studies. It is suitable for High School grade level students.