Final answer:
The length of time ice covers on Mirror Lake in New Hampshire has declined by 1 day per year.
Step-by-step explanation:
The length of time ice covers on Mirror Lake in New Hampshire has declined by 1 day per year (option b) according to the given choices. This means that each year, the duration of the ice cover on Mirror Lake is reducing by one day. It suggests a trend of shorter ice cover seasons over time.