The entire purpose of route aggregation is summarization -- replacing a set of more-specific contributing routes with a single, broader prefix advertised outward -- and this summarization inherently hides internal routing instability from external peers: if one of the underlying contributing /24 blocks within a summarized /16 flaps up and down due to a local link issue, external peers who only see the stable, unchanging /16 aggregate are completely insulated from that churn, since the aggregate itself remains active and advertised as long as at least one contributing route is present, regardless of how much internal fluctuation occurs among the individual contributors. This stability- hiding characteristic is one of the primary operational benefits that motivates using aggregate routes in the first place, confirming that statement as correct. Regarding next-hop behavior, official Juniper documentation is explicit and unambiguous:
when an aggregate route is installed in the routing table, Junos assigns it a reject next hop by default, meaning traffic matching only the aggregate (and no more-specific contributor) is dropped and an ICMP unreachable message is returned to the source; a discard next hop is available only as an optional, explicitly configured alternative when silent dropping without ICMP notification is preferred, but it is not the default. Aggregate routes categorically do not advertise their individual contributing routes; suppressing those specific, more- granular prefixes from onward advertisement in favor of the single summary is the aggregate's defining function, directly contradicting the first statement.

Aggregate routes are used for advertising summarized network prefixes. They help minimize the number of routing tables in an IP network by consolidating selected multiple routes into a single route advertisement. This approach is in contrast to non-aggregation routing, in which every routing table contains a unique entry for each route.
Therefore, option D is correct. Options A, B, and C are not correct because:
Aggregate routes can be used with both static routing and dynamic routing protocols. Aggregate routes are not automatically generated for all of the subnets in a routing table. They need to be manually configured.
Aggregate routes are not always preferred over more specific routes. The route selection process in Junos OS considers several factors, including route preference and metric, before determining the active route.

The Complete Sequence Number PDU (CSNP) is the IS-IS mechanism used to summarize the entire contents of a router's link-state database, listing the LSP ID, sequence number, checksum, and remaining lifetime for every LSP the sending router currently believes should exist in the database for that level. On broadcast (LAN) circuits, the elected Designated Intermediate System periodically transmits CSNPs to all routers on the segment as a database-synchronization mechanism; any receiving router compares the summarized list against its own local database, and any discrepancy -- an LSP it is missing entirely, or one for which it holds an older sequence number -- triggers a targeted request for the specific, updated LSP content. On point-to-point circuits, a single CSNP is exchanged once when the adjacency first comes up, serving the identical synchronization purpose without needing periodic repetition. The Partial Sequence Number PDU (PSNP), by contrast, is used to acknowledge receipt of specific LSPs or to explicitly request specific LSPs identified as missing or outdated after a CSNP comparison, making it a partial, targeted PDU rather than a complete database listing. The LSP itself is the PDU that actually carries the detailed topology and reachability information being advertised, rather than a database summary. The IS-IS Hello PDU (IIH) is used purely for neighbor discovery and adjacency maintenance and carries no link-state database summary information whatsoever.

On Juniper switches, the default VLAN is set to a VLAN ID of 1 by default. This means that all interfaces on the switch are members of VLAN until they are specifically assigned to another VLAN. Therefore, option A is correct.
The default VLAN ID can be changed. This allows network administrators to configure the switch to use a different VLAN as the default, if necessary. Therefore, option D is correct.