Translation of the original source IP address to an IP address from a user-defined address pool by shifting the IP addresses. This type of translation is one-to-one, static, and without port address translation. If the original source IP address range is larger than the IP address range in the user-defined pool, untranslated packets are dropped. https://www.juniper.net/documentation/us/en/software/junos/nat/topics/topic-map/nat-security-source-and-source-pool.html

The correct address book entries are:
173.145.5.21/255.255.255.0
203.150.108.10/24
Both of these entries represent a valid IP address and subnet mask combination, which can be used as an address book entry in a Juniper device.

The correct statement about Junos security policies is that they enforce rules that should be applied to traffic transiting an SRX Series device. Security policies control the flow of traffic between different zones on the SRX Series device, and dictate which traffic is allowed or denied. They can also specify which application and service requests are allowed or blocked. More information about Junos security policies can be found in the Juniper Networks technical documentation here: https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/security-policies-overview.html.