You have a very fragile glass vase that you need to ship to your mother for mother's day but you have no packing materials. all you have is the popcorn that you are eating. you don't see that you can use the popcorn as packing material. this is best explained by

a) functional fixedness
b) confirmation bias
c) salience

This is best explained by FUNCTIONAL FIXEDNESS

Explanation:

Functional fixedness is when we are so fixated on the fact that the object can only be used for one particular function and not any other functions. For example we need a screwdriver to remove screws and that the only function we associate with a screw driver.

Functional fixedness makes us unable to discover other functions that an object can perform. It can also hinder us from thinking out of the box.

Let us say you want to remove a screw but there is no screw driver and someone suggest that you can use a particular sharp knife to drive the screw around until it is removed , you will be hesitant to believe a knife may help you do the job also.

i believe your answer is mountains.