Correct answer choice is:
D. Reservoir.
Explanation:
Further particularly, transmission transpires when the operator vacates its reservoir or host by a portal of an outlet, is carried by some convention of transmission and penetrates through a suitable portal of entry to contaminate a responsive host. This sequence is seldom called the chain of infection.
There are six links in the chain transmission, which includes:
1. The infectious agent
2. Reservoir
3. Portal of exit
4. Mode of transmission
5. Portal of entry
6. Susceptible host.
