Answer:
A. Reservoir
Step-by-step explanation:
The person with an active infection is a reservoir as he is the source of the virus which can be transmitted to other people.
Mode of transmission is the way by which a pathogen (virus, bacteria or parasite) is transferred from one person to the other. e.g. by blood, by air
Susceptible host is the individual who is at risk of acquiring the infection.
Infectious disease is any disease that is caused by a pathogen (disease causing organism) that can be transmitted from one person to the other.