Explanation/Reference:
An HP 5900 CP can be added in an HP 5900 series IRF (Intelligent Resilient Framework) to simplify management by combining multiple physical switches into one virtual switch. An IRF fabric appears as one node or virtual switch and is accessible as a single IP address on the network. You can use this IP address to log in at any member device to manage all the members of the IRF fabric.
The HP FlexFabric 5900CP switch supports four operating modes. Standard mode is the default mode.
FCF, NPV, and Transit modes are advanced modes.
When a HP 5900 CP switch operates in the NPV/Gateway Mode, it is called as an NPV (N_Port Virtualizer) switch.
In this mode, the following HP FlexFabric 5900CP converged port connections are available:
Fibre Channel (does not apply in this question)

FCoE

- HP FlexFabric 5900CP VF_Port to a VN_Port on a node.
- HP FlexFabric 5900CP VNP_Port to a VF_Port on an FCF (NPIV) switch.
Each of the other FCF switches is assigned a domain ID. They must support NPIV.
Incorrect Answers:
A, B: Switch 5 must run in NPV mode.
C: Only Switch 5 should be in NPV mode.
References: HP FlexFabric 5900CP Solutions Configuration Guide, page 5-7
http://h20565.www2.hpe.com/hpsc/doc/public/display?
sp4ts.oid=5221896&docLocale=en_US&docId=emr_na-c04216045