The correct option is Custom installation mode.

Choose Custom installation mode when you need to deploy Azure AD Connect across several Active Directory forests because it lets you add and configure each forest individually and supply separate credentials for each. The custom mode also lets you choose which synchronization features to enable and to configure OU and attribute filtering which are required for complex, multi forest topologies.

Staged deployment is not the right choice because staged deployment describes running a secondary Azure AD Connect server in staging mode for validation or failover and it does not replace the need to use the custom installer to configure multiple forests.

Either Express or Custom Settings is incorrect because the express path cannot handle complex multi forest environments and the question asks for the installation mode you should select in a multi forest scenario which is the custom mode.

Express Settings is incorrect because express settings are intended for simple, single forest deployments and they apply default configuration that does not allow you to add additional Active Directory forests or customize synchronization settings during installation.

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