B in my opinion

I think is B

Answer A is correct! https://www.exam-answer.com/cisco/350-501-spcor/question8

I would go by option A, since it clearly points to deploying a "Full separation model" between internet access and MPLS services, not centralizing individual services or traffic type. Here you will see some implementations like VRFs to keep EGP and IGP separate: https://community.cisco.com/t5/other-service-provider-subjects/service-provide-mpls-l3vpn-and-internet-access-on-same-router/td-p/3739191

why B I think D is correct

Let me correct my previous post. We can avoid the propagation of customer subnets and default route via IGP, by enabling iBGP family ipv4/6, so putting the internet in vrf is not the only option. In this case, we can exclude A.

It should be A. eBGP is used to exchange router with external peerings on the border routers. Border routers receive peering routes and advertise manually aggregate routes (like a.b.0.0/16). In the old times, the internet was in the global routing table (not vrf) and the border routers were advertising only a default route via IGP to the rest of the network. Customer subnets (like a.b.c.d/30) were automatically redistributed into the IGP by the PE routers, so they are known by the other PEs and border routers. iBGP address-family ipv4 between border routers and PE routers was not used, so the IGP was involved for distributing the default route and customer subnets.

You can choose to separate services for different reason, but the main one is security. Actually, many important carriers have separated networks for VPN and Internet. Answer “A” has no sense, EGP and IGP always operate in a separate way. Answer “C” and “D” has no sense also.

I think the question asked is incorrect and should read "What is the reason to NOT make this decision". Then the sole correct answer is D. Because all three A/B/C are correct answers for the original question.

One of the main reasons for implementing this model is to enable EGP (Exterior Gateway Protocol) and IGP (Interior Gateway Protocol) to operate independently. EGP is used to exchange routing information between different autonomous systems (AS), while IGP is used to exchange routing information within an AS. By separating Internet access and MPLS L3 VPN services into different routing instances, it is possible to use different routing protocols for each service. This can help to optimize network performance and reduce the risk of routing conflicts and instability. Another reason for implementing a full separation model is that it allows the network operator to choose whether to separate or centralize each individual service. This means that services can be deployed in a way that best suits the needs of the organization, rather than being constrained by the limitations of a single routing instance. For example, the Internet access service may be centralized for better security, while the MPLS L3 VPN service may be separated for better performance.

@Karoly, the explanation of your link also says „ Another reason for implementing a full separation model is that it allows the network operator to choose whether to separate or centralize each individual service.“ So I go for B

I would say A would be correct.