Final answer:
Thomas Malory began and completed 'Le Morte d'Arthur' while in prison, a foundational work for the Arthurian legend in English literature.
Step-by-step explanation:
Complete the sentence: While in prison, Malory began and completed “Le Morte d'Arthur”, a work that later became one of the most important and enduring versions of the Arthurian legend. Thomas Malory wrote this seminal work during his imprisonment in the 15th century, and it was printed by William Caxton in 1485. While the exact details and length of Malory's imprisonment are unclear, his resulting work significantly impacted English literature and the portrayal of the Arthurian legends.