StorageGRID custom alert rules define their trigger conditions using Prometheus expressions. NetApp StorageGRID 12.0 documentation states that when creating a custom alert rule, the administrator enters a Prometheus expression for one or more alert severity levels. These expressions evaluate metrics collected from StorageGRID nodes and services and determine when an alert condition becomes true.
For this requirement, the administrator would construct an expression that evaluates both CPU and memory utilization metrics and combines the conditions logically so that the rule triggers only when CPU usage exceeds 90% and memory utilization exceeds 80% for the same applicable node. StorageGRID's Prometheus service collects time-series metrics from services throughout the grid, and those metrics are used by both the Grid Manager monitoring views and the alerting framework.
Option A is incorrect because the Grid Management REST API can expose metrics and can be used to test Prometheus queries, but it is not the syntax used to define the alert condition itself. Option B is an S3 object- query capability and is unrelated to system monitoring. Option C is not the alert-expression language.
Therefore, D. Prometheus expression is the correct answer.
Reference topics: Grid Manager # Alerts # Alert Rules # Create Custom Rule # Prometheus Metrics and Expressions # CPU/Memory Monitoring