Final answer:
Shawn should buy 6 buckets of paint for his fort.
Step-by-step explanation:
To find out how many buckets of paint Shawn should buy, we need to calculate the total area he needs to paint. The total surface area of the inside of the fort can be found by multiplying the height by the sum of the lengths of all four sides. In this case, the surface area is 6 feet x (6 feet + 6 feet + 6 feet + 6 feet) = 6 feet x 24 feet = 144 square feet. Since each bucket of paint covers 300 square feet, Shawn would need to buy at least 1 bucket of paint. Therefore, the answer is A. 6.