Which is considered the earliest substantial written document in Old English?

Explanation

Option A refers to Boethius's philosophical writings, which are not Old English texts. Option B is a Latin Bible translation by Saint Jerome, unrelated to Old English. Option C, Malory's Morte Darthur, is a Middle English work. The correct answer is D, the legal code established by King Ethelbert, recognized as the earliest extended text written in Old English.