If the chloroplast is the result of endosymbiosis, what membrane of the chloroplast represents the plasma membrane of the original prokaryote?

The inner membrane of chloroplast

Explanation:

According to the endosymbiotic theory chloroplast and mitochondria is evolved from endosymbiosis of prokaryotic cell by an ancestral cell.  

So the prokaryotic cell is engulfed through endocytosis. So the aerobic bacteria must be engulfed through internalization of the plasma membrane. The invaginated plasma membrane then budded off inside the ancestral cell and made the outer layer of chloroplast.

Therefore the inner membrane of bacteria represents the plasma membrane of the original prokaryote and the outer membrane of chloroplast represents the plasma membrane of ancestral cell.