Ans is option (B)
Let the speed of the boat be $v$ miles per hour.
While going upstream, river current opposes the boat’s motion. So, the effective speed of the boat would be $(v-2)$ miles per hour.
$\therefore$ $1.5=\frac{3}{v-2}$ $\Rightarrow$ $v=4$ miles per hour.
While going downstream, river current supoorts boat motion. So, the effective speed of the boat would be $(v+2)=4+2=6$ miles per hour.
$\therefore$ time taken for the downstream trip $=$ $\frac{3}{6}=0.5$ hours $=$ $30$ minutes