Final answer:
The term that describes a great difference between the intended purpose of an action and its opposite result is A) Irony, as seen in literary works like Shakespeare's Romeo and Juliet.
Step-by-step explanation:
The term that describes "a great difference between the purpose of a particular action and the result; the desirable outcome turns out to be the opposite of what is wanted" is A) Irony. Irony often presents itself when the actions taken have an effect exactly opposite to what was intended, making the outcome surprising or unexpected. It can be found in many narrative forms, including literature and plays. For instance, a classic example of irony can be seen in Shakespeare's Romeo and Juliet, where the character's actions to secure their love lead to tragic consequences, the opposite of their intentions.
Reversal is also an important term in plot analysis, signifying an unexpected twist that changes the course of events. An example of a reversal is when a character's actions inadvertently create a complication that obstructs their goals, like the undelivered letter in Romeo and Juliet. However, reversal does not encapsulate the complete meaning of the term we're looking for about actions that produce contrary results to what was intended.
Climax and falling action are stages in a narrative but don't describe the phenomenon of unexpected outcomes as irony does.