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
The most abundant prokaryotes helpful to humans in making curd from milk and in the production of antibiotics are heterotrophic bacteria. Heterotrophic bacterial are useful to humans in a variety of ways. But they are also known to cause various harmful diseases. Hence the correct option is (4).