Some species of present day protists contain living organisms within their cytoplasm. Paramecium bursaria are hosts for zoochlorellae, photosynthetic protists, that live within the cytoplasm of the paramecium. The relationship appears to be symbiotic; the zoochlorellae gain protection and possibly some essential nutrients from the host cytoplasm and the paramecium host has an available food source when its usual food source is depleted. This symbiotic relationship provides support for the theory of endosymbiosis, which states that A) animal-like cells evolved from ancestors that were photosynthetic. B) mitochondria and chloroplasts evolved from small prokaryotes living within larger cells. C) photosynthetic organisms, like today's land plants, evolved from photosynthetic bacteria. D) heterotrophic organisms could not have evolved before the evolution of photosynthetic organisms.

Endosymbiotic theory is an evolutionary theory that describes the origin of mitochondria and chloroplast (cell organelles of eukaryotes) from the prokaryotes.

These two organelles share characteristics with bacteria as they both have circular DNA, capable of self replication, transcription, and translation.

It is beleived that prokaryotes used to live in symbiotic association with larger cells and later they developed into these organelles (particularly "alpha proteobacteria" developed into 'mitochondria' and "cyanobacteria" developed to '' 'chloroplast').

According to this information, option B) is the right answer for theory of endosymbiosis.