Final answer:
Kelly Nolin's reaction to finding the flag suggests that she recognized its significance, leading us to infer that she knew the flag was very important, which is option A.
Step-by-step explanation:
When Kelly Nolin experienced a quiet moment and her heart skipped a beat upon finding the flag, we can infer that A. she knew the flag was very important.
This reaction is commonly associated with discovering something of significance or of emotional value.
The intensity of her experience suggests a recognition of the flag's importance rather than options B, C, or D, which are excitement due to the Civil War, secrecy of her work, or being in a scary place, respectively.