**Core Concept**
Prevalence is a measure of the total number of cases of a disease present in a population at a given time. It is calculated as the product of the incidence rate and the mean duration of the disease.

**Why the Correct Answer is Right**
To calculate prevalence, we can use the formula: Prevalence = Incidence x Mean Duration. Given that the incidence is 2 cases per 1000 population per year and the mean duration is 4 years, we can plug in these values to find the prevalence. The correct calculation is Prevalence = 2 cases/1000/year x 4 years = 8 cases/1000 population.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option does not provide a calculation, so it cannot be correct.
* **Option B:** Without the mean duration, we cannot accurately calculate the prevalence.
* **Option C:** This option would be correct if the mean duration were 1 year, but not with a mean duration of 4 years.
* **Option D:** This option would be correct if the incidence were 8 cases/1000/year, but the incidence is actually 2 cases/1000/year.

**Clinical Pearl / High-Yield Fact**
When calculating prevalence, remember that it is the product of incidence and mean duration. This can be a useful formula to remember for epidemiological studies.

**Correct Answer: C. 8 cases/1000 population.**