Which weld symbol represents a fillet weld?

The question asks which weld symbol represents a fillet weld.

• A. Triangle symbol $\triangle$: This is the standard symbol used to denote a fillet weld on a welding drawing.
• B. Circle symbol $\bigcirc$: This symbol typically indicates a spot weld or a weld that is to be made all around the joint.
• C. Square symbol $\square$: This symbol is used for a square groove weld.
• D. Flag symbol $\text{P}$: A flag symbol on a welding drawing indicates that the weld is a field weld, meaning it is to be performed at the site of construction rather than in the shop. It does not represent a type of weld.

Therefore, the triangle symbol represents a fillet weld.

The final answer is $\boxed{\text{A. Triangle symbol }\triangle}$