Answer: ingestion of a cyanobacterium which remained intact and evolved into a chloroplast. Cyanobacteria evolved oxygenic photosynthesis.
Explanation: The eucaryote cell presents this picture.
An archaean cell ingested an aerobic bacterium which remained intact and the two co-evolved into cell plus mitochondria. A cell like this ingested a cyanobacterium which remained intact and evolved into a chloroplast. It is still unclear how the eucaryote nucleus, multiple chromosomes and membrane bound organelles, e.g. Golgi apparatus and endoplasmic reticulum evolved.