Step 1: Identify which phase you want the steel in.
For forging to be easy and defect free, plain carbon steel needs to be fully austenitic, since austenite is soft, ductile and offers low resistance to flow compared to the harder ferritic or mixed phase structure at lower temperatures.
Step 2: Set the safe upper and lower bounds.
The lower bound must stay comfortably above the A3 line on the iron carbon diagram so the steel remains fully austenitic and workable, while the upper bound must stay well clear of the solidus line, because forging too close to melting can cause grain boundaries to begin melting, a defect called burning that ruins the material.
Step 3: Match the practical range to the given values.
For ordinary plain carbon steels, industry practice keeps the forging temperature roughly between 1100 and 1300 degrees Celsius, giving enough margin below the solidus while keeping the steel comfortably soft. Among the listed values, 1300 degrees Celsius sits right at this practical upper working limit.
\[ \boxed{1300^\circ C} \]