Step 1: Understand the type of vaccine.
Covishield is a viral vector vaccine. A modified adenovirus carries the gene for the SARS-CoV-2 spike protein into human cells, where the spike protein is then made and trains the immune system. We must explain why the adenovirus cannot copy itself inside us.
Step 2: Recall what a virus needs to replicate.
To multiply, a virus needs its own genes for copying its genome, making viral proteins, and assembling new particles. Remove those genes and the virus can still deliver cargo but cannot reproduce.
Step 3: See how the vector is engineered.
During design, the adenovirus's replication genes are deleted, and the spike protein gene is slotted into that empty space. This makes it a replication-deficient vector, safe but still able to express the spike protein once.
Step 4: Test option (A).
Adenoviruses infect animal cells, not bacteria, so the idea that it only grows in bacterial populations is simply wrong. (A) is rejected.
Step 5: Test options (C) and (D).
Attenuation (C) means a weakened but still living and replicating organism, which is not how Covishield is made safe. A killed-virus vaccine (D) contains inactivated particles, which is also not this design, since the vector is alive but replication-deficient. Both are rejected.
Step 6: Confirm option (B).
The accurate reason is that the genes for growth and replication were removed and replaced by the spike protein coding gene, which is the definition of a replication-deficient adenoviral vector. So (B) is correct.
\[ \boxed{\text{Genes involved in growth and replication have been replaced by the spike protein coding gene.}} \]