Bacteria are :
(a) free-living organisms
(b) parasites
(c) nitrogen-fixers
(d) all of these

Options

A. free-living organisms
B. parasites
C. nitrogen-fixers
D. all of these

Answer

To solve the question "Bacteria are:", we need to analyze each of the options provided:

1. **Option (a): Free-living organisms**
- Bacteria can exist independently in various environments. They are capable of living freely in soil, water, and other habitats. An example of a free-living bacterium is Azotobacter, which plays a role in the ecosystem.

2. **Option (b): Parasites**
- Some bacteria are indeed parasitic, meaning they depend on a host organism for survival and reproduction. An example is the bacteria that cause strep throat. These bacteria invade and live off the host's resources.

3. **Option (c): Nitrogen-fixers**
- Certain bacteria have the ability to fix nitrogen, converting atmospheric nitrogen into a form that plants can use. This can include both free-living bacteria like Azotobacter and symbiotic bacteria like Rhizobia, which live in association with plant roots.

4. **Option (d): All of these**
- Since bacteria can be free-living, parasitic, and nitrogen-fixing, this option encompasses all the previous statements.

After analyzing all the options, we conclude that the correct answer is **(d) all of these**.