What observation led researchers to propose that chloroplasts evolved from cyanobacteria

Photosynthesis

Explanation:Researchers thought that both chloroplast and Cynobacteria preform photosynthesis as common and hence proposed that chloroplast was evolved from Cynobacteria. Chloroplast are one of the most specialized structures present in plant and algal cells. It is a membrane bound organell and its function is to perform photosynthesis by capturing sunlight with the help chlorophyll pigments(present in chloroplast) and convert it into energy(ATP).Cyanobacteria are also known as blue green algae because they are  blueish green in color due of the presence of photosynthetic pigments in it.Cyanobacteria comes under bacteria but they are capable of trapping the sunlight and produce energy without chloroplast. Which shows that they must have some connection with chloroplast.

Result: Cynobacteria are considered as the ancestors of chloroplast.