In a Binary Search Tree $($BST$),$ what property holds true for all nodes?

a$.$ There is no specific order between parent and child nodes

b$.$ The left child is always smaller than the parent, and the right child is always greater

c$.$ The left child is always smaller than or equal to the parent, and the right child is always greater

d$.$ The left child is always greater than the parent, and the right child is always smaller