Final answer:
Option (A), GRE (Generic Routing Encapsulation) is designed to transport various protocols but has limited security features, while IPSec provides strong security features for IP traffic only. GRE over IPSec is a combination that brings together the advantages of both by enabling the transportation of any protocol with strong security over an IP network.
Step-by-step explanation:
Because the focus of GRE (Generic Routing Encapsulation) is to transport many different protocols, it has very limited security features. By contrast, IPSec has strong data confidentiality and data integrity features, but it can transport only IP traffic. GRE over IPSec combines the best features of both protocols to securely transport any protocol over an IP network. GRE allows the encapsulation of a wide variety of network layer protocols inside virtual point-to-point links over an IP internetwork. On the other hand, IPSec provides authentication and encryption for the secure transport of packets. Combining GRE with IPSec enables the advantage of protocol diversity of GRE along with the encryption strength of IPSec.