What was the experimental organism used by Frederick Griffith?

The correct answer is Option (3) → Streptococcus pneumoniae

In 1928, Frederick Griffith worked with the bacterium Streptococcus pneumoniae, which causes pneumonia. Its S strain has a mucous coat, forms smooth colonies and kills mice; the R strain has no coat and does not cause disease. Griffith found that heat-killed S strain bacteria mixed with live R strain bacteria could kill mice, because the R strain had been transformed by some ‘transforming principle’ from the dead S strain. This principle was later shown to be DNA.

NCERT Extract:
The Search for Genetic Material – “… In 1928, Frederick Griffith, in a series of experiments with Streptococcus pneumoniae (bacterium responsible for pneumonia), witnessed a miraculous transformation in the bacteria. During the course of his experiment, a living organism (bacteria) had changed in physical form. When Streptococcus pneumoniae (pneumococcus) bacteria are grown on a culture plate, some produce smooth shiny colonies (S) while others produce rough colonies (R).”