Tough one jodyw. I am not familiar with the American grade system but I would guess that you are looking for something fairly general/basic. It is a big subject as you will see from previous entries on this forum and so the job is to pick out a few points and show that you know something about them.
If you say that gardens gave warrior lords a place to rest you will then explain that there was a lot of conflict within society. You will probably talk about samurai. You could say that the Japanese garden started as a protected area away from warfare.

A mention of Muso Soseki as the inventor of the dry garden (just stones and gravel)helps to explain that Buddhist priests were the first to bring the garden style from China to Japan...

Here's a link that might help:

Here is a link that might be useful: Japanese garden