The correct answer is C.
The word “along” is the most appropriate word to use in this sentence because it describes the path that the speaker and the person leading them took. The speaker was led along the green lawns to the palatial building.
The word “upon” is not a good choice because it suggests that the speaker and the person leading them were on top of the green lawns.
The word “across” is not a good choice because it suggests that the speaker and the person leading them went from one side of the green lawns to the other.
The word “about” is not a good choice because it suggests that the speaker and the person leading them were moving in a circular or irregular path around the green lawns.