**Core Concept**
Incidence is a measure of the number of new cases of a disease within a specified time period among a population at risk. In this context, the population at risk refers to the individuals who were not already myopic on January 1, 2011.

**Why the Correct Answer is Right**
To calculate the incidence of myopia, we need to identify the population at risk, which is the number of individuals who were not already myopic on January 1, 2011. This is calculated as the total population minus the number of individuals who were already myopic, which is 5000 - 500 = 4500. The number of new myopia cases in 2011 is given as 90. Therefore, the incidence of myopia in 2011 is calculated as the number of new cases divided by the population at risk: 90 / 4500 = 0.02 or 2%.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not account for the population at risk. If we simply divide the number of new cases by the total population, we get an incorrect incidence rate.
* **Option B:** This option is incorrect because it is not a valid calculation for incidence. Incidence is a rate, not a proportion or percentage.
* **Option C:** This option is incorrect because it is a proportion, not a rate. Incidence is typically expressed as a rate per 1000 or 10,000, not as a proportion.

**Clinical Pearl / High-Yield Fact**
When calculating incidence, it's essential to identify the population at risk and use the correct formula to avoid errors.

**Correct Answer: D. 0.02 or 2%**