Explanation:

Let’s break down the problem:

- Let speed of boat in still water = 4x, speed of current = 3x

- Downstream speed = (4x + 3x) = 7x

- Upstream speed = (4x – 3x) = x

- Distance covered downstream = 126 km, upstream = 126 km

- Time for downstream = 126/7x = 18/x hours

- Time for upstream = 126/x hours

- Total time: 18/x + 126/x = 144/x = 48 hours

- So, x = 144/48 = 3 km/hr

- Speed of boat in still water = 4x = 12 km/hr

- Option 1: 16 km/hr – Incorrect

- Option 2: 9 km/hr – Incorrect

- Option 3: 10.5 km/hr – Incorrect

- Option 4: 3 km/hr – Incorrect

- Option 5: 12 km/hr – Correct