Answer: new_cluster

The `new_cluster` field is mandatory in the payload when creating a job through the Databricks REST API because it specifies the cluster configuration on which the job will run. This includes details like the instance type and number of instances, which are crucial for the job's execution. While fields like `job_name`, `notebook_task`, and `email_notifications` are useful for job management and execution details, they are not mandatory. The `job_name` helps in identifying the job, `notebook_task` defines the task the job will execute, and `email_notifications` are for setting up alerts, but none of these are required for the job creation process.

Author: LeetQuiz Editorial Team