Architecture such as this demonstrates which of the following

I would say the correct answer is the power of the medieval Church.
Its power was so great and it was so rich that it could pay people to work on building new churches which were so elaborate and beautiful as this one. This is an example of a cathedral, so I assume A is the correct answer, having nothing to do with monarchs, universities, or merchant guilds.

To help prevent any one of the 3 branches from gaining too much power.