Final answer:
Without additional information on the relationship between Leon's and his nephew's ages, the correct equation to determine the nephew's age when Leon is 21 cannot be ascertained.
Step-by-step explanation:
The equation to find the nephew's age (n) when Leon is 21 years old is represented by one of the provided options. The correct option must account for the relationship between Leon's age and his nephew's age. To decide which equation is correct, we would need additional information about how Leon's age and his nephew's age are related. If, for instance, Leon is always 4 years older than his nephew, we would choose an equation that reflects this situation, such as Leon's age = nephew's age + 4.
Without this context, none of the options can be determined as the correct equation. Therefore, it is important to have clear details on the relationship between their ages to solve for the nephew's age.