The correct answer is C. church.
Music and the minds of people during the Middle Ages were mostly influenced by religion, the church. Secular music was still not a thing - it refers to worldly music, not related to religion. Musical instruments were used to perform music, but they did not actually inspire it. Humanism came after the Middle Ages - the Middle Ages lasted until the 15th century, and humanism started in the 15th, 16th centuries.
However, church was omnipresent at the time - everyone was religious, and if they were not, they were considered heathens. Therefore it is only natural that religion would influence most spheres of life during the Middle Ages.