According to the official Fortinet administration guide for FortiOS 7.6.4 under the section "Configuring a RADIUS server," the supported RADIUS authentication methods you can configure via the CLI with config user radius are:
pap
chap
mschap
mschapv2
auto
The relevant CLI syntax is set auth-type {auto | ms_chap_v2 | ms_chap | chap | pap}. You can confirm this directly in the configuration table and from real CLI sessions.
EAP (Extensible Authentication Protocol) is NOT an authentication option you can directly set under config user radius. EAP methods (such as EAP-TLS, EAP-PEAP, EAP-TTLS) are negotiated between the RADIUS client and server but are not configurable as an explicit auth-type option in FortiOS. EAP authentication is typically used automatically by features like 802.1X, not through the user radius object authentication-type setting, and always requires proper backend workings between supplicant and RADIUS server

To understand why Type 5 LSAs (AS External LSAs) are missing from the Link-State Database (LSDB), we must look at how OSPF generates and propagates them:
A). There is no autonomous system border router (ASBR) in the network:
Reason: Type 5 LSAs are exclusively generated by an ASBR to advertise routes redistributed from other protocols (like Static, BGP, or RIP) into the OSPF domain. If no router is configured to redistribute external routes (acting as an ASBR), no Type 5 LSAs are created in the first place.
C). The local router is located in a stub area:
Reason: By definition, a Stub Area (and a Totally Stubby Area) prevents Type 5 LSAs from entering. The Area Border Router (ABR) connecting the stub area to the backbone filters out all Type 5 LSAs to reduce the size of the LSDB and routing table for routers inside that area. Instead, a default route is usually injected.
Why other options are incorrect:
B: While database filtering exists, standard prefix-list filtering typically affects the routing table (RIB) generation, not the underlying LSDB propagation of Type 5 LSAs, or it is less common than the architectural reasons (Stub/No ASBR).
D: IP Protocol 89 is the transport for OSPF itself. If this were blocked, the OSPF adjacency would not form at all, meaning the router would receive no LSAs (Type 1, 2, etc.), not specifically just Type 5.
Reference:
FortiGate Security 7.6 Study Guide (OSPF): "Type 5 LSAs are generated by ASBRs... Stub areas do not allow Type 5 LSAs; they are replaced by a default route."