To find the power developed by the boy, we first need to determine the force he applies to lift the mass. Since the mass is lifted at a constant velocity, the force applied must be equal to the weight of the mass. Step 1: Calculate the weight of the mass. The weight ($W$) of an object is given by the formula: $$W = m \times g$$ where $m$ is the mass and $g$ is the acceleration due to gravity. Given: Mass ($m$) = $29 \text{ kg}$ Acceleration due to gravity ($g$) = $10 \text{ m/s}^2$ $$W = 29 \text{ kg} \times 10 \text{ m/s}^2$$ $$W = 290 \text{ N}$$ Since the mass is lifted at a constant velocity using a single pulley system, the force ($F$) the boy applies is equal to the weight of the mass. $$F = 290 \text{ N}$$ Step 2: Calculate the power developed. Power ($P$) is defined as the product of force and velocity when the force is applied in the direction of motion: $$P = F \times v$$ Given: Force ($F$) = $290 \text{ N}$ Velocity ($v$) = $5 \text{ m/s}$ $$P = 290 \text{ N} \times 5 \text{ m/s}$$ $$P = 1450 \text{ W}$$ Comparing this result with the given options: A. 40 W B. 100 W C. 250 W D. 1450 W The calculated power matches option D. The final answer is $\boxed{\text{D. 1450 W}}$.