Step 1: Recall the family tree.
D is A's grandparent (grandfather, since D is B's father), B and C are the married couple and A's parents, and A is the grandchild. Checking each person's two statements against this tree: A's both statements are false, B's both statements are true, C has one true and one false statement, and D has one true and one false statement.
Step 2: Check option A.
D is A's grandparent, and D is established as male (grandfather), so D cannot be A's “grandmother” at all; this option does not even describe a real person in the tree, so it cannot be the statement that must be true.
Step 3: Check option B.
C is female and already established as B's wife; C does not have a “wife” of her own, so this statement describes a relationship that does not exist in this family, and cannot be true.
Step 4: Check option C.
D is A's grandfather, and D's two statements were found to be one true and one false, not both true, so D does not always speak the truth; this option is false.
Step 5: Check option D.
B's daughter is A, since B and C are A's parents. A's two statements were both found to be false, meaning A always lies. This matches exactly.
Step 6: Final Answer.
The statement that must be true is option D, B's daughter always tells lies.