A node in a binary tree has degree $0, 1$ or $2$.
Ref: http://faculty.cs.niu.edu/~mcmahon/CS241/Notes/bintree.html
We are given no. of $1$ degree node $= 5$, no. of $2$ degree nodes $= 10$.
Total no. of edges $= 1*5 + 2*10 = 25$ (In tree degree is for outgoing edges only, and hence each degree corresponds to an edge)
So, total no. of nodes $= 25 + 1 = 26$ (No. of nodes in a tree is $1$ more than no. of edges).
Now, no. of leaf nodes (nodes with $0$ degree) $= 26 - 5 - 10 = 11$.
Correct Answer: $B$