Which temperature transducer has these characteristics?
Remember with multi-choice questions the instruction is to choose the answer which you think is best,

PTC thermistor is a generic term that includes metal RTDs and Polyswitches, -  anything with a mostly positive slope.

The curve on that graph is surely some sort of PTC device 
that two RTD answers do not match this curve, leaving 
PTC thermistor as the best choice.