Answer:
a) 100 per 100,000
Explanation:
The term Incidence refers to the number of individuals who develop a specific disease during a particular time period (a month, a year, etc) in a certain population. Thus, incidence refers to the new cases that present in a population over the course of some specific amount of time.
In this case, there were 100 NEW cases of syphilis last year in a population of 100,000 people.
Since incidence refers to only the new cases of a disease we really don't need to focus on the 500 people who already had syphilis.
Thus, the incidence of syphilis in this population is a) 100 per 100,000
