Final answer:
A perineal laceration that extends into the rectal mucosa after childbirth is classified as a fourth-degree laceration (option D).
Step-by-step explanation:
The perineal laceration described extends into the rectal mucosa, which indicates a significant tear occurring during the birth of a full-term baby. The classification of perineal tears in childbirth has four degrees, where first-degree involves only the perineal skin, second-degree extends into the muscles beneath the skin, third-degree reaches the muscles around the rectum, and fourth-degree affects the rectal sphincter and the rectal mucosa. Given that the laceration reaches the rectal mucosa, this situation is documented as a fourth-degree laceration.
Hence, the answer is option D.