What is the primary characteristic of a bilateral body plan?

The primary characteristic of a bilateral body plan is that organisms exhibit mirrored halves along a single plane, meaning that the left side and the right side of the body are often nearly symmetrical. This arrangement allows for greater mobility and is conducive to streamlined movement, which is advantageous for animals that are active and need to navigate through their environment efficiently.

Bilateral symmetry supports the development of a head and tail region, leading to the concentration of sensory organs and nervous tissues at one end (cephalization), which is a significant evolutionary advantage. This body plan is seen in a wide variety of organisms, including mammals, birds, and insects, making it a fundamental aspect of animal morphology and evolution.

In contrast, options involving symmetry along multiple axes or around a central point describe other symmetry types, such as radial symmetry, where body parts are arranged around a central axis. The option indicating "no symmetry" refers to asymmetrical body structures, which do not fit the definition of bilateral symmetry at all.