Final answer:
To find the percentage of rangers working solely in Central Park, divide the number of Central Park rangers (4) by the total number of city rangers (20) and multiply by 100 to get 20%.
Step-by-step explanation:
To calculate what percentage of rangers work solely in Central Park, we use the information that 4 out of 20 rangers are assigned to Central Park. By dividing the number of Central Park rangers by the total number of rangers and then multiplying by 100, we get the percentage of rangers working in Central Park.
Calculation: (4 ÷ 20) × 100 = 20%
Therefore, the correct answer is: a) 20%