What is the prime condition of AA-tree which makes it simpler than a red-black tree?

What is the prime condition of AA-tree which makes it simpler than a red-black tree?
 
(a) Only right children can be red
 
(b) Only left children can be red
 
(c) Right children should strictly be black
 
(d) There should be no left children
 
My question is from Binary Trees topic in section Binary Trees of Data Structures & Algorithms I
 
I have been asked this question in an interview for job.