Which one of these facts about mitochondria and chloroplasts constitutes support for the endosymbiotic theory?
a. their dna resembles bacterial rna.
b. their ribosomes resemble the ribosomes of bacteria.
c. they are organelles that are found within plant cells.
d. they reproduce by mitosis.

The correct answer will be option B (Their ribosomes resemble the ribosomes of bacteria).

Explanation:

Endosymbiotic theory was proposed by Biologist Lynn Margulis in the 1960's which stated that a large cell ingested the bacteria for survival which became dependent on one another, resulting in a permanent relationship which cannot live outside the cell.

Mitochondria and chloroplasts were evolved from the endocytosis of prokaryotes aerobic and photosynthetic bacteria respectively.

Both mitochondria and chloroplasts posses their own DNA and 70S ribosomes which shows similarity to the DNA and ribosomes of bacteria.

Thus, option B is the correct answer.