The f(x) and g(x) are closed under addition because when added, the result will be a polynomial , Option D is the correct answer.
The missing polynomial are attached as an image in the answer.
The closure Property states that , when an addition or any other property is said to be closed , it means the output will be same as the type of input.
When the polynomial given in the question is added the resulting output will always be a polynomial , therefore
The f(x) and g(x) are closed under addition because when added, the result will be a polynomial , Option D is the correct answer.
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