Answer: Option B. -> Blue-green algae were the first photosynthetic organisms on Earth.
:
B and C

All microbes except certain fungi cannot be seen or observed with the naked eye. They can only be observed using a microscope. Blue-green algae, also known as cyanobacteria, were the first photosynthetic organisms on Earth. Penicillin and streptomycin are antibiotics obtained from the fungus Penicillum and the bacterium Streptomyces respectively. 
 

Answer: Option B. -> Streptomycin
:
A, B, and C
Penicillin, streptomycin and tetracycline are antibiotics. Penicillin is obtained from Penicillium notatum, and tetracycline and streptomycin are obtained from Streptomyces. Pepsin is a protein-digesting enzyme found in the stomach.

Answer: Option D. -> Malaria
:
D
  $\bullet$ Protozoans are eukaryotic, unicellular microorganisms. Most of them are free-living, found in soil and fresh water, while some are parasitic and cause diseases in plants and animals. Malaria is caused by the protozoan parasite, Plasmodium.
  $\bullet$ Tuberculosis (TB) is caused by a type of bacterium called Mycobacterium tuberculosis.
  $\bullet$ Polio is a highly contagious infectious disease caused by polio virus.
  $\bullet$ Typhoid fever is an acute infectious illness associated with fever that is most often caused by the Salmonella typhi bacteria.

Answer: Option C. -> Aedes mosquito
:
C

Dengue is caused by a virus and is spread by a species of mosquito called Aedes. The spread of this disease can be prevented by using mosquito nets, repellents and by control of breeding of mosquitoes by not allowing water stagnation in our surroundings.

Answer: Option C. -> Both the statements are correct.
:
C

Viruses are infectious entity which are seemingly alive only inside a host cell.
They are non-living outside the living cell. They are composed of genetic material (DNA or RNA) but never both enclosed within a protein coat called capsid. They lack cellular organisation due to the absence of cell organelles, plasma membrane, etc. Viruses cause many infectious diseases including polio, herpes and common cold.

Answer: Option C. -> Weak microorganisms
:
C
A vaccine is a biological preparation containing weakened or dead microbes. It is introduced into a healthy person's body so that it can produce antibodies against the disease-causing microbes. Vaccines can be injected or administered orally.

Answer: Option B. -> Lactobacillus
:
B
Lactobacillus is the microorganism present in curd. This bacterium converts milk into curd at 30 to 40${}^o$C.

Answer: Option D. -> pathogens
:
D
Pathogens is the term given for harmful disease-causing microorgansims. They can enter our body through contaminated air, water or food. They can also get transmitted by direct contact with an infected person or carried through an animal. Bacteria, fungi, protozoa, etc are examples of microorganisms that can be pathogens.

Answer: Option A. -> True
:
A
Boiling is an important method of making food contaminant-free. Boiling food kills the microbes present in the food. When food is boiled at high temperatures, microbes present cannot survive in those conditions.