The endosymbiont theory states that chloroplasts originated a) from cyanobacteria entering and remaining in the cytoplasm of another cell. b) through evolutionary change in a mitochondrion. c) by fusion of several prokaryotes. d) by spontaneous generation in the cytoplasm of an ancestral cell. e) by fusion of mitochondria and peroxisomes.

The endosymbiotic theory states that chloroplast originated from A Cyanobacteria entering and remaining in the cytoplasm of another cell.

Explanation:

Endosymbiotic theory in an important theory of evolution which states that Eukaryotic cells are originated from prokaryotic organism.

        The theory deals with the fact that chloroplast and mitocondria were free living prokaryotic organism at Past. The similarities between prokaryotic cell and these Eukaryotic organelles(Mitocondria,Chloroplast) and the evolution of mitocondria and chloroplast from the prokaryotic cells is best exlained by the theory of endosymbiosis.

Mitocondria evolved from the Endocytosis of aerobic bacteria,whereas Choroplast are originated from photosynthetic bacteria such as cyanobacteria.

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!