The correct option is A Lederberg and Tatum
Lederberg and Tatum (1946) discovered bacterial conjugation as a mode of sexual reproduction in bacteria. Conjugation is transfer of genetic material between two bacterial strains via conjugation tube. Wollman, Hayes and Jacob (1952) showed presence of different mating types/sexual differentiation in bacterium due to presence/absence of sex factors. Griffith (1928) discovered transformation as mode of sexual reproduction in bacteria that is transfer of genetic material from one bacterial strain to other without establishing a physical contact. Since, there is no physical contact established during transformation, it did not established presence of sexuality in bacteria despite being discovered earlier than conjugation. Loeb and Zinder (1961) discovered RNA containing bacteriophages. Correct answer is option A.