Which of the following processes requires contact between a donor cell and a recipient cell?

A. Heterotroph: Heterotrophs obtain carbon from organic compounds; if this organism were a heterotroph it would require organic carbon in the medium (or be able to digest agar or other organics), which the description does not indicate.

B. Autotroph: Autotrophs fix carbon dioxide as their carbon source; increased growth under high CO₂ and growth on a mineral salts medium with no digestible organics strongly suggest a CO₂-fixing autotroph.

C. Chemotroph: Chemotrophs obtain energy from chemical compounds rather than light; this term describes energy source rather than carbon source and can overlap with autotrophy or heterotrophy, so the information given points more directly to carbon fixation (autotrophy).

D. Halophile: Halophiles require or tolerate very high salt concentrations for growth; although the medium contains salts, nothing indicates extreme salinity preference, and the CO₂ response points toward autotrophy rather than halophily.

A. F+ cells: F+ cells contain the fertility (F) plasmid that encodes the genes required for pilus formation and DNA transfer during conjugation.

B. F− cells: F− cells lack the F plasmid and serve as recipients, not initiators, of conjugation.

C. All bacterial cells: Only cells possessing the F plasmid can initiate conjugation; therefore, not all bacterial cells are capable of this process.