Answer:
D, The use of spolia in monuments like the arch of constantine was considered to be a decline of form compared to earlier roman architecture
Step-by-step explanation:
I know B is wrong, as it actually depicts a victory over Maxentius at the Milvian Bridge than a war against the Dacians. I believe the Use of Spolia is considered to be a decline as is taken materials/art from other monuments dedicated to other people and reusing it, showing either lack of resources or lack of creativity, hence a decline in either of the things.