Final answer:
Chloroplasts are derived from photosynthetic cyanobacteria through endosymbiosis, supporting option a. Plastids, including chloroplasts, evolved through an endosymbiotic process. DNA sequences confirm the close relationship between chloroplasts and cyanobacteria, supporting option c.
Step-by-step explanation:
Chloroplasts are derived from once free-living photosynthetic cyanobacteria, which supports option a. Evidence from genetic and morphological studies suggests that plastids, including chloroplasts, evolved through endosymbiosis. The endosymbiotic hypothesis proposes that an ancestral cell engulfed a photosynthetic cyanobacterium, which eventually evolved into modern-day chloroplasts. Furthermore, the DNA sequences of the chloroplasts in plants are closely related to cyanobacteria, providing additional support for option c.