Explanation:

- Let's denote the speed of the boat in still water as $b$ and the speed of the stream as $s$.

- When going downstream, the boat's speed is $b+s$, and upstream it's $b-s$.

- Given: Downstream speed is thrice of upstream speed. Therefore, $b+s=3(b-s)$.

- Simplifying, $b+s=3b-3s$ leads to $4s=2b$, hence $b=2s$.

- If the speed of the stream is halved, new speed $=\frac{s}{2}$.

- New downstream speed becomes $b+\frac{s}{2}$.

- Time to travel 50 km now is 4 hours: $\frac{50}{b+\frac{s}{2}}=4$.

- Solving $50=4(b+\frac{s}{2})$, implies $b+\frac{s}{2}=12.5$.

- Replace $b$ with $2s$, hence $2s+\frac{s}{2}=12.5$, results in $\frac{5s}{2}=12.5$.

- Solving $5s=25$ leads to original $s=5$ km/hr.

The correct option is: Option 1: 5 km/hr