Answer:
Step-by-step explanation:
The correct usage would be:
"By then, her children were grown, and she had fewer responsibilities."
In this sentence, "fewer" is the appropriate comparative form of the adjective "few," which should be used when describing things that can be counted, such as responsibilities. Additionally, there should not be a double comma after "grown" - only one comma is needed to separate the introductory phrase from the main clause.