Inorder successor of a node is a minimum element in right subtree of that node. Here $15's$ right subtree having $17$ as minimum element which is a inorder successor of $15$.
Morever if we write inorder of the given tree:
2, 3, 4, 6, 7, 9, 13, 15, 17, 18, 20 element after 15 is inorder successor
Option c) is correct