Answer:
D. the training of infectious disease researchers.
Step-by-step explanation:
The source would help a reader get more knowledge about the training that clinicians and researchers get regarding infectious diseases. We can see that the reader will gain more understanding of the infectious diseases training in professionals because the email details the institution's name, which gives a general idea about their mission. Also, the message mentions the things that this organization does: improve infected patient care, training, and control of infectious diseases.
There is no mention in the email about the other options. The only thing that the message mention is the ISID's objectives, where the training of professionals regarding infectious diseases is one of them. In conclusion, the reader will gain more understanding about option d.