Which feature should be used with a static route that has a secondary next hop...

In Junos, a static route can be configured with multiple next hops. When you want a “primary and backup” behavior, the correct mechanism is a qualified next hop. A qualified next hop allows you to assign a different preference to each next hop for the same destination prefix. The static route with the best preference is selected as active, while the higher-preference value next hop remains available as a standby option. If the primary next hop becomes unusable, Junos can install the backup qualified next hop so forwarding continues with minimal operational change, which is a common requirement for data center edge or services routing where predictable failover is needed.

The key point in the question is “secondary next hop with a unique route preference value.” That wording maps directly to qualified next-hop behavior, because qualification is how Junos differentiates multiple next hops under the same route using distinct preference values. The retain option relates to keeping routes in the routing table under certain conditions and does not specifically create primary/backup next-hop selection based on preference. The resolve option concerns how the system resolves a next hop through another route and is not the feature that creates a preference-ranked secondary next hop. The install option is not the mechanism used to define backup next-hop preference for a static route.