A boat can move $35\text{km}$ upstream and the same distance downstream in a total time of $8$ hours. If the speed of the boat in still water is $9\text{km/h}$, then the speed (in $\text{km/h}$) of the stream is:

Let the speed of the stream be $v\text{km/h}$.
Given:
* Speed of the boat in still water ($u$) = $9\text{km/h}$
* Downstream speed $=u+v=9+v$
* Upstream speed $=u-v=9-v$
* Distance ($d$) = $35\text{km}$

According to the question, total time taken for both journeys is $8$ hours:
$$\frac{35}{9-v}+\frac{35}{9+v}=8$$

Let's test the given choices to find which value satisfies the equation:
Try *Option B ($v=2$)**:
$$\frac{35}{9-2}+\frac{35}{9+2}=\frac{35}{7}+\frac{35}{11}=5+3.18\ne 8$$

Try *Option D ($v=3$)**:
$$\frac{35}{9-3}+\frac{35}{9+3}=\frac{35}{6}+\frac{35}{12}$$
$$\frac{70}{12}+\frac{35}{12}=\frac{105}{12}\ne 8$$

Try *Option A ($v=2.5$)**:
$$\frac{35}{9-2.5}+\frac{35}{9+2.5}=\frac{35}{6.5}+\frac{35}{11.5}\ne 8$$

Let's verify the exact algebraic solving if there's any typo or look for the exact integer root:
$$35\left(\frac{(9+v)+(9-v)}{(9-v)(9+v)}\right)=8$$
$$35\left(\frac{18}{81-v^2}\right)=8$$
$$\frac{630}{81-v^2}=8$$
$$630=8(81-v^2)$$
$$630=648-8v^2$$
$$8v^2=648-630$$
$$8v^2=18$$
$$v^2=\frac{18}{8}=\frac{9}{4}=2.25$$
$$v=\sqrt{2.25}=1.5\text{km/h}$$

Thus, Option C ($1.5$) perfectly satisfies the solution.