Which of the following is characterized as the "6 pack" muscle?

The rectus abdominis is commonly referred to as the "6 pack" muscle due to its appearance when well-defined and strong. This muscle runs vertically along the front of the abdomen and is responsible for flexing the spinal column, contributing to movements like bending forward. When a person has low body fat and engages in exercises that target this muscle, segments or fibers within it become more visible, creating the characteristic six-pack look.

The other options refer to muscles that play different roles in core stability and movement. The transversus abdominis acts as a stabilizer for the spine and pelvis and does not have a visual appearance associated with a "6 pack." The internal and external obliques are involved in rotational movements and lateral flexion of the trunk but do not form the segmented look that the rectus abdominis does. Thus, the rectus abdominis is recognized for its prominent '6 pack' look, making it the correct answer in this context.