Final answer:
The piece of evidence that best supports the struggle to conform is the excerpt that describes the character's sister instinctively bristling when she saw the nuns approaching. So, the correct answer is option c.
Step-by-step explanation:
The piece of evidence that best supports the struggle to conform is option C) "when our oldest sister saw the nuns approaching, she instinctively bristled." This excerpt suggests that the character's sister is resistant to conforming to the expectations imposed by the nuns, indicating a struggle to conform.