Complete the sentence with the most appropriate word or phrase out of the given options.
The release of the book was planned to ____ with the writer's fiftieth birthday.

coincide

The most appropriate word to complete the sentence is coincide.

The other options are not as accurate. Harmonize suggests that the release of the book was planned to be in harmony with the writer's fiftieth birthday, but this is not the case. Fall with suggests that the release of the book was planned to happen after the writer's fiftieth birthday, but this is also not the case. Synchronize suggests that the release of the book was planned to happen at the same time as the writer's fiftieth birthday, but this is not the most accurate description. Coincide is the best option because it suggests that the release of the book was planned to happen on the same day as the writer's fiftieth birthday.