Let the possible families be
$BB,\ BG,\ GB,\ GG$
where $B$ stands for boy and $G$ stands for girl.
Now one boy comes into the room.
So the possible cases are:
From $BB$: either of the two boys may come.
$\Rightarrow (BB,\text{ first boy}),\ (BB,\text{ second boy})$
From $BG$: only the boy can come.
$\Rightarrow (BG,\text{ boy})$
From $GB$: only the boy can come.
$\Rightarrow (GB,\text{ boy})$
Thus, total possible cases $=4$
Out of these, the favourable cases in which the other child is also a boy are:
$(BB,\text{ first boy}),\ (BB,\text{ second boy})$
So favourable cases $=2$
Hence,
$P(\text{the other child is also a boy})=\dfrac{2}{4}=\dfrac{1}{2}$
Therefore, the correct option is C.
