B. They could not participate in the existing French legislature.
The estates would meet periodically at an assembly at the Estates-General. At the assemblies, the third estate did not hold a lot of power and was at a disadvantage. The first and second estates would exercise their powers to veto any proposals.
The third estate (commoners) were locked out, so, they relocated to the Royal Tennis Court to discuss a potential rebellion, this was known as the Tennis Court Oath.