Final answer:
The key of "for unto us a child is born" is D major.
Step-by-step explanation:
In Handel's oratorio "Messiah," the famous chorus "For unto us a child is born" is in the key of D major. This piece is a celebration of the prophecy and birth of Jesus Christ, and Handel masterfully composed it in a triumphant and uplifting tone, using D major to convey a sense of joy and grandeur fitting for the subject matter. D major, with its two sharps (F# and C#), carries a bright and majestic quality, often associated with triumph and exultation, making it a fitting choice for this powerful proclamation of the birth of a significant figure in Christian belief.
Correct answer: The key of "for unto us a child is born" is D major.