A

if (key < root->val)

  insert(root->left, key);

else if (key > root->val)

  insert(root->right, key);

B

if (key < root->val)

  insert(root->right, key);

else if (key > root->val)

insert(root->left, key);

C

insert(root->left, key);

insert(root->right, key);

D

insert(root->right, key);

insert(root->left, key);