The main climax in the novel Pride and Prejudice, by Jane Austen, is the moment when Darcy proposes to Elizabeth. Surely there are several moments of suspense and intensity in the novel, but a climax can be identified as the most pivotal situation in the story and the moment after which the rest of the action begins to fall: Secrets are often revealed after the climax, and problems "magically" find a solution.
The proposal is pivotal to the story because the "brutally honest" way in which Darcy and Elizabeth speak to each other during its delivery unveils that their mutual antipathy is a result of many misconceptions.
This is a red flag that all of those misconceptions are about to be resolved as a result of their being brought up during the proposal. This confirms that the proposal is, in fact, the main climax of the story.
To further validate this assumption, notice how the falling action can be identified right after the proposal. First, Darcy writes to Elizabeth a letter in which he gives her the entire truth about Wickham. Right after this happens, Elizabeth learns that Wickham eloped with her youngest sister, which gives validity to Darcy's words. From then on, Elizabeth begins to change, Darcy also changes, and situations begin to find their solution.
Conclusively,the proposal is the catalyst that unveils most of the misconceptions expressed by the main characters, and is also the agent that helps those misconceptions become clarified.