Final answer:
The episiotomy extending into the rectal sphincter and mucosa is classified as a 4th degree laceration, the most severe involving the an*l sphincter and the rectal mucosa (Option D).
Step-by-step explanation:
The classification of the laceration in question, where an episiotomy has extended into the rectal sphincter and mucosa, is a 4th degree laceration. The different degrees of perineal lacerations during childbirth are categorized by the extent of the tissue involvement.
A 1st degree laceration involves only the vaginal mucosa or perineal skin, a 2nd degree laceration extends into perineal muscles and vaginal mucosa, a 3rd degree laceration extends into the an*l sphincter, and a 4th degree laceration extends through the an*l sphincter into the rectal mucosa. In this case, the extension to the rectal mucosa indicates a 4th degree laceration.
Thus, the correct option is D.