A weld with triangular cross sections is used to join two surfaces at right angles to each other, and is called a __________.

Correct Answer: Option D — fillet weld

A fillet weld is characterized by a triangular cross-section used to join two surfaces at approximately right angles, such as in lap joints or T-joints. It is the most common weld type for structural steel applications and piping attachments where full penetration is not required.