According to the theory of endosymbiosis, the origin of chloroplasts probably involved:
a) the formation of colonies of cyanobacteria.
b) the formation of cell walls around the photosynthetic pigments.
c) engulfing of small photosynthetic prokaryotes by a larger cell.
d) the accumulation of free oxygen in ocean waters.
e) infolding of the cell membrane around circular pieces of dna in the cytoplasm.

Option C  

Explanation:

Engulfing of the small photosynthetic by a larger cell. The theory of the endosymbiotic states that the ingested bacteria  and the host cell can easily become dependent on each other for the survival which results in the permanent relation.

Over millions of year, the mitochondria and the chloroplast have become more specialized and they cannot live outside the cell.

Hence, the correct answer is option C