The correct answer is option 3: Turtle.

From an evolutionary point of view, the term "primitive" refers to an animal that has retained more ancestral characteristics or traits compared to other animals. In this case, the turtle fits this definition because it has retained many primitive features that were present in its ancestors. Turtles have a shell, which is a unique and distinct feature not found in many other animals. This shell is believed to have evolved millions of years ago and has remained relatively unchanged, making turtles one of the most primitive animals in existence.

Option 1: Dolphin - Dolphins are not considered primitive animals. They belong to a group of mammals called cetaceans, which have evolved specialized adaptations for swimming and life in water.

Option 2: Otter - Otters are also not considered primitive animals. They are mammals belonging to the family Mustelidae and have evolved adaptations for life in aquatic environments.

Option 4: Walrus - Walruses are also not considered primitive animals. They are large marine mammals and have evolved a variety of specialized adaptations for survival in Arctic environments.

Overall, the most primitive animal among the options provided is the turtle, as it has retained many ancestral characteristics over millions of years of evolution.