Final answer:
Jim Halpert proposes to Pam Beesly at a rest stop during a rainstorm in 'The Office' S5E1 'Weight Loss'.
Step-by-step explanation:
In S5E1 "Weight Loss" of The Office, Jim Halpert proposes to Pam Beesly at a rest stop during a rainstorm. This heartfelt scene occurs right as they are about to part ways again with Pam going back to art school in New York and Jim returning to Scranton. Jim surprises Pam (and the audience) by getting down on one knee with a gas station's convenience store as their backdrop, highlighting that it's the relationship and not the setting that truly matters.