In a representative stress-strain plot for metals above (taken from wikipedia),
1: Ultimate tensile strength
2: Yield strength (yield point)
4: Strain hardening region
5: Necking region
A: Apparent stress (F/A0)
B: Actual stress (F/A)
A is the engineering stress-strain curve as it relies on the original area of a tensile test specimen rather than taking into account the changing area of the specimen. (1) is indicating the ultimate tensile strength, and so the maximum stress.
Therefore, the answer choice is $C$.