For checking the Reporting element is in range you will do checking of 2 nodes at each height .
And Between the you can have “k” nodes which will encounter in the counting range .
So total element ( node ) for reporting all element in range [a,b] will be : ( k + 2*h) where k: nodes in the range , 2*h : Total Number of nodes which you will check at each height .
Height ( h) = logn ( BST )
So, worst overall complexity Ɵ(k+2*logn)