The correct answer is A. It supports a maximum of two domains. Juniper's Active Directory identity-source configuration guidance states that an SRX Series Firewall using the integrated user firewall/Active Directory identity source can be configured for a maximum of two domains. That is the exact limit relevant to this question. The same Juniper configuration page separately states that a maximum of 10 domain controllers can be configured, which directly eliminates option C because the supported figure is not 20 Active Directory servers per domain.
Option B is wrong because Active Directory identity source is not presented as a logical-systems feature in the JNCIS-SEC identity-aware firewall context. Option D is wrong because the primary Active Directory mechanism reads Windows domain controller event logs and uses WMI/DCOM plus LDAP to build IP-to- user and group mappings. Juniper describes the SRX as reading and monitoring the Windows event log on the domain controller, then using LDAP to obtain user/group information. Non-domain or non-Windows cases typically require alternate authentication behavior such as captive portal, not native Active Directory event- log tracking.
Reference topics: Identity-Aware Security Policies, Active Directory identity source, integrated user firewall, domain limits, domain controllers, WMIC, LDAP mapping.