Genetic engineering , also called genetic modification or genetic manipulation is the direct manipulation of an organism's genes using biotechnology. The vaccines produced through genetic engineering are considered as safe because the antigen is taken from the coat of pathogen. Conventional vaccines contain the whole unit of attenuated or killed pathogen.
So, the correct answer is ' They contain antigen only from the coat of pathogen.

Introduction:
Vaccines produced through genetic engineering have revolutionized the field of immunization by offering safer and more effective alternatives. These vaccines are designed to elicit an immune response without causing the disease they prevent. Among the options provided, option 'A' is the correct answer. Let's explore why.

Explanation:
Vaccines produced through genetic engineering are safe because they contain antigens only from the coat of the pathogen. Here's a breakdown of each option and why option 'A' is the correct choice:

a) They contain antigen only from the coat of the pathogen:
- Genetic engineering techniques allow scientists to isolate and insert specific genes encoding the antigens present on the pathogen's coat into the vaccine.
- These genes are then expressed by the recipient's cells, leading to the production of the corresponding antigens.
- By including only the antigens from the pathogen's coat, the vaccines avoid any potential risks associated with using the entire pathogen or its live form.
- This approach ensures that the vaccine is safe while still providing the immune system with the necessary information to recognize and respond to the pathogen.

b) They are the least active form of the virus:
- This statement is not entirely accurate and may introduce confusion.
- Vaccines produced through genetic engineering do not necessarily involve using the virus itself.
- Instead, they utilize specific components or antigens of the pathogen, such as proteins or polysaccharides.
- The term "least active form of the virus" is not applicable to genetic engineering-based vaccines.

c) They are attenuated forms of the pathogen:
- This statement is not accurate for vaccines produced through genetic engineering.
- Attenuated vaccines are made from live viruses or bacteria that have been weakened, so they can still replicate but cause minimal or no disease.
- Genetic engineering-based vaccines do not involve the use of attenuated forms of the pathogen, as they typically focus on specific antigens rather than the entire pathogen.

Conclusion:
In summary, vaccines produced through genetic engineering are safe because they only contain antigens from the coat of the pathogen. This approach ensures that the immune response is targeted and effective while minimizing the risk of adverse effects associated with using the whole pathogen or its attenuated form.