Read this and answer the question. If the question is unanswerable, say "unanswerable".

Shoen holders had access to manpower and, as they obtained improved military technology (such as new training methods, more powerful bows, armor, horses, and superior swords) and faced worsening local conditions in the ninth century, military service became part of shoen life. Not only the shoen but also civil and religious institutions formed private guard units to protect themselves. Gradually, the provincial upper class was transformed into a new military elite based on the ideals of the bushi (warrior) or samurai (literally, one who serves).

What institutions were forbidden to form private guards?
unanswerable