What is the first step when preparing to install ES?

According to the Splunk Enterprise Security documentation, the first step when preparing to install ES is to determine the size and scope of installation. This involves estimating the amount of data that you plan to ingest, the number of users that will access ES, the number of search heads and indexers that you need, and the hardware requirements for each component. This step helps you plan your deployment architecture and ensure optimal performance and scalability of ES. Therefore, the correct answer is D. Determine the size and scope of installation. References = Deployment planning.