The most abundant prokaryotes helpful to humans in making curd from milk and in production of antibiotics are the ones categorised as:
Cyanobacteria
Archaebacteria
Chemosynthetic autotrophs
Heterotrophic bacteria
As learnt
Heterotrophic bacteria are most abundant in nature. Many of them have a significant impact on human affairs. These are helpful in making curd from milk (e.g., Lactobacillus sp.), production of antibiotics (e.g., Streptomyces sp.) and fixing nitrogen in legume roots (e.g, Rhizobium spe).
Option 1)
Cyanobacteria
This option is incorrect
Option 2)
Archaebacteria
This option is incorrect
Option 3)
Chemosynthetic autotrophs
This option is incorrect
Option 4)
Heterotrophic bacteria
This option is correct