Which type of VPN involves a nonsecure tunneling protocol being encapsulated by IPsec?

The correct choice involves Generic Routing Encapsulation (GRE) being combined with IPsec. GRE is a tunneling protocol that can encapsulate a variety of network layer protocols within a point-to-point link. However, GRE does not provide encryption by itself, making it nonsecure. This is where the use of IPsec comes into play, as it provides the necessary security features such as encryption and authenticity to the GRE traffic.

By encapsulating GRE within IPsec, the data packets can travel securely across the Internet or any potentially untrusted network. This combination effectively allows for secure transmission of GRE packets, ensuring that sensitive information is protected while still utilizing the tunneling capabilities of GRE.

The other options listed do not correctly depict this specific combination of protocols. For instance, while L2TP does offer some benefits, it typically requires an additional security layer like IPsec to provide encryption, which does not match the criteria presented in the question.