Final answer:
The best array declaration for storing retail prices is 'Declare real retailPrices', as it allows for decimal precision necessary for representing prices.
Step-by-step explanation:
To store retail prices which often include a decimal point to represent cents, the appropriate data type would be a real or floating-point data type. Among the options provided, b) Declare real retailPrices is the best suited for this purpose. Integer and string data types are not well-suited for representing prices as integers do not allow decimal precision and strings would not allow for mathematical calculations without conversion.