Final answer:
When Darlene chooses to sunbathe instead of working, she is giving up the opportunity to earn $20.
Step-by-step explanation:
The trade-off Darlene faces when she decides to spend two hours sunbathing in the afternoon instead of working at her job that pays $10 per hour is the opportunity cost of the $20 she could have earned working for those two hours (option B). By choosing to sunbathe, Darlene is giving up the potential earnings from her job.