Ans) d 

Exp) Option d is correct. 

ACE2 (angiotensin-converting enzyme 2) is a protein on the surface of many cell types. It is an enzyme that generates small proteins – by cutting up the larger protein angiotensinogen – that then go on to regulate functions in the cell. Using the spike-like protein on its surface, the SARS-CoV-2 virus binds to ACE2 – like a key being inserted into a lock – prior to entry and infection of cells. Hence, ACE2 acts as a cellular doorway – a receptor – for the virus that causes COVID-19. 

In the search for treatments for COVID-19, many researchers are focusing their attention on a specific protein that allows the virus to infect human cells. 

Source) https://theconversation.com/what-is-the-ace2-receptor-how-is-it-connected-to-coronavirus-and-why-might-it-be-key-to-treating-covid-19-the-experts-explain-136928 

 Subject) Science