Correct answer is A.
The couple has two children and one is known to be boy,
The probability that the other is boy will be
\(=\frac{Favorable\,outcomes}{Total\,outcomes}\)
Total outcomes are 3,
The first child is a boy, the second girl
The first child is a girl, the second boy
The first child is a boy, second boy
The favourable outcome is one,
Thus, the probability that the other is boy will be
\(\Rightarrow 1/3\)