Final answer:
The opening movement of Bach's cantata 'A Mighty Fortress Is Our God' is best described as a chorale, a musical composition harmonizing a Lutheran hymn tune for voices, and often used in church services. The correct answer to the question is A. Chorale.
Step-by-step explanation:
The opening movement of Johann Sebastian Bach's cantata 'A Mighty Fortress Is Our God' is best described as a chorale. A chorale is a musical composition that harmonizes a Lutheran hymn tune, often written for four voices. Bach's cantatas frequently include chorales that are based on well-known hymn tunes, serving as an integral part of the church services for which they were composed. This style of music was particularly prevalent in the Baroque period, during which Bach composed a vast array of liturgical music.
In the context of Bach's cantatas, a recitative typically refers to a vocal solo that advances the narrative, while an aria refers to a more expressive and melodic piece, usually also a solo. A sinfonia, on the other hand, is an instrumental introduction or interlude. Given these definitions, the correct answer to the question is A. Chorale.