.......... is an antiviral substance produced by animals in response to viral infection.

The correct option is D Interferon

• When the immune system is activated due to the viral infection or other immune response the white blood cells in our body produces Interferons (are a group of proteins called cytokines).Interferons suppress the viral multiplication.
  
• The name of the interferons comes from their ability to interfere with the production of new virus particles. Interferons do not directly kill viral or cancerous cells; they boost the immune system response and reduce the growth of cancer cells by regulating the activity of several genes that control the secretion of numerous cellular proteins that affect growth.
  
• Hence Interferon is an antiviral substance produced by animals in response to viral infection'.
• So, the correct answer is 'Interferon'.