B

Step 1: Determine the weight of the block. The weight W of the block is calculated using the formula: W = mg, where m = 1 kg and g = 9.8 m/s². Thus, W = 1 kg * 9.8 m/s² = 9.8 N.
Step 2: Identify the net force acting on the block. The boy is pulling down with a force of 5 N. Hence, the net force F_net = W - 5 N = 9.8 N - 5 N = 4.8 N (acting downward on the block).
Step 3: Apply Newton's second law to find the acceleration of the block. Using F_net = ma, we have 4.8 N = 1 kg * a. Thus, a = 4.8 m/s².
Step 4: Calculate the distance the block falls before it can no longer be seen. The height of the boy's eyes from the ground is 1 m, and since the image of the block can still be seen until it reaches the same height, the block will fall 1 m.
Step 5: Use the equation of motion to find the time t taken to fall 1 m. We know that h = ut + (1/2)at², where initial velocity u = 0, h = 1 m, and a = 4.8 m/s².
1 = 0 + (1/2)(4.8)t². Simplifying, we have t² = 1/(2.4) = 0.4167, hence t = √0.4167 ≈ 0.645s.
Step 6: Therefore, the time during which the image of the block can be seen by the boy is approximately 0.645 seconds.