Which sentence is grammatically correct?

The sentence "Sarah plays her new saxophone well." is grammatically correct because it follows standard English sentence structure and usage. The subject, "Sarah," performs the action of the verb "plays," and "her new saxophone" correctly identifies what she is playing. Additionally, the adverb "well" effectively modifies the verb "plays," indicating how she plays the saxophone.

In contrast, the other sentences contain grammatical issues. For instance, using "good" as a modifier in a few of the options does not align with standard usage, as "good" is an adjective and should be replaced with "well," an adverb, to correctly modify a verb. The final choice, while it uses "well" appropriately, lacks clarity on what "well" is intended to describe.