(A) Dogen Zenji, the founder of the Sōtō school of Zen Buddhism, conducted rituals in his monastery, they included.
This option suffers from a lack of clarity. The pronoun "they" is ambiguous and does not clearly refer to the rituals.
(B) Rituals were conducted in Dogen Zenji, the founder of the Sōtō school of Zen Buddhism’s monastery, which included.
This option has an unclear sentence structure. It suggests that the rituals were conducted in the monastery, but it is not clear who conducted them.
(C) Dogen Zenji, who was the founder of the Sōtō school of Zen Buddhism in his monastery, conducted rituals including.
This option has an awkward sentence structure. The phrase "who was the founder of the Sōtō school of Zen Buddhism in his monastery" disrupts the flow of the sentence and is not necessary for conveying the intended meaning.
(D) Dogen Zenji, the founder of the Sōtō school of Zen Buddhism, conducted rituals in his monastery that included.
This option is the most suitable choice. It introduces Dogen Zenji as the subject and provides necessary information about him being the founder of the Sōtō school of Zen Buddhism. The phrase "conducted rituals in his monastery that included" effectively conveys the actions performed by Dogen Zenji.
(E) In his monastery, Dogen Zenji, the founder of the Sōtō school of Zen Buddhism, conducted rituals that included.
This option is fairly similar to option (D), but the placement of the introductory phrase "In his monastery" can create ambiguity. It may suggest that the monastery itself conducted the rituals, rather than Dogen Zenji.
In summary, option (D) provides a clear and concise sentence structure that accurately conveys the intended meaning without introducing unnecessary information or ambiguity.