READ QUESTION VERY CAREFULLY :
2 main lines 1)Every Node has odd number of descendants(Childs)
2)Every node considered to be its own descendants(Parent will be considered as it's own child)
As this is binary tree ,Only 2 cases will be possible.
1.Parent with No child (In this case 1(odd) descendants , which is parent itself).
2.Parent with 2 Childs (In this case 3(odd) descendants (1 Parent +2 Child).
NO NODE WILL BE WITH 1 CHILD SO ANSWER IS 0.