Final answer:
In Duncker's candle experiment, the subjects experienced functional fixation, which is the cognitive bias that prevents individuals from perceiving alternative uses for an object beyond its conventional function. The correct option is 1.
Step-by-step explanation:
In Duncker's candle experiment, the obstacle to problem-solving that the subjects experienced was functional fixation. Functional fixation is a cognitive bias where individuals perceive an object to be used only in its conventional way and are unable to think of alternative uses for that object.
In the experiment, participants were given a candle, matches, and thumbtacks and were instructed to attach the candle to the wall without it dripping wax onto the table below. Many participants were fixated on the functional use of the thumbtacks as attaching the candle directly to the wall, overlooking the creative solution of emptying the box and using it as a candle holder.
Hence, Option 1 is correct.