Geometry

Question Analysis
This question involves identifying the incorrect biconditional statement.
The main focus is on verifying the logical validity of each statement.

Key Concept Explanation:
A biconditional statement requires both the original statement and its converse to be true, linked by "if and only if."
If either part is false, the biconditional statement is incorrect.

Step-by-Step Solution:
Analyze each option to determine if both the original statement and its converse are true.

Option A:
Original: If a quadrilateral is a rhombus, then all its sides are equal. (True)
Converse: If a quadrilateral has all sides equal, then it is a rhombus. (True)
This is a valid biconditional statement.

Option B:
Original: If a triangle is right-angled, then it has one 90° angle. (True)
Converse: If a triangle has one 90° angle, then it is right-angled. (True)
This is a valid biconditional statement.

Option C:
Original: If an angle is obtuse, then it measures more than 90° but less than 180°. (True)
Converse:...