Final answer:
The muscle that attaches to the 12th ribs and fixes their positions during expiration is the a. internal oblique muscle.
Step-by-step explanation:
The muscle that attaches to the 12th ribs and fixes their positions during expiration is the a. internal oblique muscle.
The internal oblique is a flat, intermediate abdominal muscle that has fascicles running perpendicular to those of the external oblique.
When the internal oblique muscle contracts, it draws the ribs together, aiding in expiration.