The mode of transmission in the chain of infection refers to the method an infectious agent uses to move from one host to another.
To clarify the other options:
- "The path by which an infectious agent leaves its host" describes the portal of exit.
- "The people and animals at risk of becoming hosts of infection" describes the susceptible host.
- "The habitat in which an infectious agent lives, grows and multiplies" refers to the reservoir.
Therefore, the correct description of the mode of transmission is: The method an infectious agent uses to move.