Most abundant prokaryotes helpful to humans in making curd from milk and in production of antibiotics are the ones categorized as

The correct option is heterotrophic bacteria.

To understand why this is the correct classification, let us break down the biological characteristics and roles of these organisms step-by-step:

1. Classification by Nutrition:
Prokaryotes (bacteria) can be broadly classified based on how they obtain their food and energy:
- Autotrophs synthesize their own organic food from inorganic substances. They can be photosynthetic (like cyanobacteria, which use sunlight) or chemosynthetic (which use chemical energy).
- Heterotrophs cannot synthesize their own food. Instead, they depend on external organic nutrients from other organisms or organic matter for nutrition.

2. Role in Making Curd from Milk:
The production of curd from milk relies on lactic acid bacteria (LAB), such as Lactobacillus. These bacteria ferment the lactose sugar present in milk into lactic acid, which coagulates the milk proteins. Since they obtain energy by breaking down the organic sugars present in milk, they are classified as heterotrophic bacteria.

3. Role in the Production of Antibiotics:
A vast majority of naturally occurring antibiotics are derived from soil-dwelling bacteria, particularly species belonging to the genus Streptomyces and Bacillus. These organisms decompose organic matter in their environment to absorb nutrients, which also makes them heterotrophic bacteria.

4. Abundance:
Heterotrophic bacteria are the most abundant prokaryotes in nature and play critical roles as decomposers, as well as in various human industries (like dairy and medicine) due to their diverse metabolic capabilities.