Final answer:
The correct answer is option b. Darcy's statement to Elizabeth about not performing to strangers refers to their reserved social interactions with strangers, not their musical abilities, family backgrounds, or true personalities.
Step-by-step explanation:
When Darcy says to Elizabeth, "We neither of us perform to strangers," after listening to her piano playing, he is referring to option B) their social interactions with strangers. This comment highlights the characters' reticence to reveal their true selves or fully engage in their talents in the presence of people they do not know well.
It's a shared trait between Darcy and Elizabeth that reflects on their personalities and how they choose to interact, or more accurately, not interact, in social situations with those they consider strangers.