The concept of endosymbiosis (chloroplast & mitochondria) is widely accepted. What kind of relationship is it?

mutualistic

Explanation:

The interaction between two organisms in which both the organism gets benefitted from each other is called a mutualistic relationship. This mutualistic relationship is seen in the concept of endosymbiosis.

According to the endosymbiotic theory chloroplast and mitochondria are evolved by engulfment of different bacteria by ancestral cells. Then these bacterial remained in a mutualistic relationship with each other and the ancestral cell evolved into a eukaryotic cell.

So as mitochondria and chloroplast live with a symbiotic relationship with eukaryotic cells, therefore, it is considered as mutualistic relationship.