When Is the "Pr" Region Cleaved by the Protease Furin

When is the "pr" region cleaved by the protease furin from the prM protein of flaviviruses?

A) During translation of the structural proteins.
B) When the virion acquires the envelope from the ER.
C) As the capsid is being assembled in the cytoplasm.
D) After the virion moves through the trans Golgi network.
E) After release of the virions from the host cell.