The answer to the question: What is the title of the work that reaches its triumphant ending in the key of D major? is inside the article: Handel's music for Messiah  is distinguished from most of his other oratorios by an orchestral restraint—a quality which the musicologist Percy M. Young observes was not adopted by Mozart and other later arrangers of the music. The work begins quietly, with instrumental and solo movements preceding the first appearance of the chorus, whose entry in the low alto register is muted. A particular aspect of Handel's restraint is his limited use of trumpets throughout the work. After their introduction in the Part I chorus "Glory to God", apart from the solo in "The trumpet shall sound" they are heard only in "Hallelujah" and the final chorus "Worthy is the Lamb". It is this rarity, says Young, that makes these brass interpolations particularly effective: "Increase them and the thrill is diminished". In  "Glory to God", Handel marked the entry of the trumpets as da lontano e un poco piano, meaning "quietly, from afar"; his original intention had been to place the brass offstage (in disparte) at this point, to highlight the effect of distance.  In this initial appearance the trumpets lack the expected drum accompaniment, "a deliberate withholding of effect, leaving something in reserve for Parts II and III"  according to Luckett.Although Messiah is not in any particular key, Handel's tonal scheme has been summarised by the musicologist Anthony Hicks as "an aspiration towards D major", the key musically associated with light and glory. As the oratorio moves forward with various shifts in key to reflect changes in mood, D major emerges at significant points, primarily the "trumpet" movements with their uplifting messages. It is the key in which the work reaches its triumphant ending. In the absence of a predominant key, other integrating elements have been proposed. For example, the musicologist Rudolf Steglich has suggested that Handel used the device of the "ascending fourth"  as a unifying motif; this device most noticeably occurs in the first two notes of "I know that my Redeemer liveth" and on numerous other..., can you guess it ?
Messiah