Final answer:
A host is an organism that harbors a pathogen with little or no immunity to it, differentiating it from a vector or an active carrier. The correct answer is B. Host.
Step-by-step explanation:
An individual into whom a pathogen has been transmitted and who has little or no immunity to infection by that organism is a host. A host is an organism that harbors a pathogen and can suffer from the disease caused by the pathogen. This individual differs from a vector, which transmits pathogens from one host to another, often without being infected by the pathogen itself, and from an active carrier, an infected individual who can transmit the pathogen to others.
The term reservoir refers to a long-term host of a pathogen, which typically does not get affected by the pathogen, making it a source of the pathogen for new individuals. Given the options provided, the correct answer to the student's question is B. Host, as it fits the description of an individual with little or no immunity to the pathogen that has been transmitted to them.