The paramecium is a single-celled, eukaryotic protist that survives and thrives today. based on current theory, from which organism did the paramecium and all other single-celled organisms evolve? a) plants b) bacteria c) yeast cell d) another protist

Ans. (B) bacteria.

Prokaryotes are most primitive organisms in the earth. According to endosymbiotic theory, eukaryotic cells are evolved from primitive, prokaryotic cells.

It states that the eukaryotic cell evolved as the eukaryotic organelles, such as chloroplasts and mitochondria are originated from symbiotic bacteria, such as cynobacteria and proteobacteria.  

Hence, it can be concluded that the paramecium and other single-celled eukaryotic organisms are evolved from bacteria.

Thus, the correct answer is option (B).