Select the most appropriate phrasal verb to fill in the blank.

To focus on writing his upcoming book, the head astronomer decided to ________ the daily management of the observatory to his senior assistant.

The correct answer is hand over.

Explanation:
The sentence requires a phrasal verb that means to pass control, responsibility, or authority of something to another person.

Let's analyze the meanings of the given options:

1. hand over: Means to transfer control, authority, or management of something to someone else. Here, the head astronomer is delegating the daily management of the observatory to his senior assistant.
2. stand by: Means to support someone or to be ready for action.
3. break into: Means to enter forcibly or to successfully enter a profession/field.
4. call off: Means to cancel a planned event or activity.

Therefore, hand over is the most appropriate phrasal verb to fill in the blank.