correct answer would be (C).
No of binary tree possible with $6$ node =$\frac{\binom{2n}{n}}{n+1}$=$\large \frac{\binom{12}{6}}{7}=132$
No of min heap possible with $6$ node = $\large \binom{5}{3}*\binom{2}{1}*\binom{1}{1}=20$
Due to symmetry ,No of max heap possible with $6$ node = $\large \binom{5}{3}*\binom{2}{1}*\binom{1}{1}=20$
So ,with 6 nodes total number of min/max heap possible =$(20+20)=40$
So ,Probability that a binary tree is a heap = $\large \frac{40}{132}=\frac{10}{33}$
