**Core Concept:** Measles is a highly contagious viral disease that affects the respiratory system and causes a rash. The secondary attack rate is the probability of a susceptible individual getting an infection when exposed to an infected person.

**Why the Correct Answer is Right:** The correct answer is derived from understanding the concept of attack rate and its calculation. In this case, the primary case refers to a person with measles, and the relatives are the susceptible individuals. The attack rate formula is: (number of susceptible individuals exposed / number of infected individuals) x 100%. In this scenario, 10 susceptible relatives are exposed and 6 of them develop the disease. Therefore, the secondary attack rate is calculated as (6/10) x 100% = 60%.

**Why Each Wrong Option is Incorrect:**
A. This option does not provide a numerical calculation based on the given data.
B. This option provides a wrong numerical value (60% is incorrect) and does not explain the calculation process.
C. This option is incorrect as it does not follow the attack rate formula and does not explain the calculation process.
D. This option provides a wrong numerical value (60% is incorrect) and does not explain the calculation process.

**Clinical Pearl:** The secondary attack rate provides valuable information about the transmissibility of the disease in a given population and helps in formulating strategies to control the spread of infectious diseases.

**Correct Answer:** Using the attack rate formula, the correct answer is **60%**. The secondary attack rate of measles is approximately 60%. This value indicates the high transmissibility of measles, emphasizing the need for vaccination programs to control its spread in the community.