The question below is from my English exercise book
Choose the one option - a, b, c or d - that best fits each of the numbered blank.
David Evans is a farmer. He does farming on his own land in Wales. The farm has belonged his family for generations. The soil is poor. David raises sheep on his land. He does not employ any full-time workers. His sons help him when they are not at school. His wife adds to the family income by selling eggs which her hens produce. In summer she often takes paying guests into the farmhouse. The guests from the city enjoy their quiet holiday in the clean country air and Mrs. Evans enjoys having some (1)___ money to spend on clothes for her children and herself.
a) more b) free c) extra d) interest
I can easily eliminate option b) & d) but I just can't understand why option c) is chosen as the correct answer by the book. Does "have some more money to spend on something" sound natural to native English speakers?