A passphrase is a type of protection that encrypts RSA keys when they are exported and imported. A passphrase is a sequence of characters that the user enters to decrypt the key. The passphrase acts as a symmetric key that is used to encrypt and decrypt the RSA key with a symmetric algorithm, such as AES. This way, the RSA key is protected from unauthorized access or tampering when it is transferred or stored. A passphrase can also provide additional security by adding entropy to the RSA key generation process. A file, NGE, and nonexportable are not types of protection that encrypt RSA keys when they are exported and imported. A file is a container that stores the RSA key, but does not encrypt it. NGE stands for Next Generation Encryption, which is a set of cryptographic standards and algorithms that Cisco recommends, but it is not a specific type of protection. Nonexportable is a property that prevents the RSA key from being exported at all, but it does not encrypt it. References: RSA/Schannel Key BLOBs, Common Encryption Types, Protocols and Algorithms Explained, Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0 (Module 5: Implementing Secure Communications with VPNs, Lesson 5.1: Implementing Site-to-Site VPNs, Topic 5.1.2: Implementing Site-to-Site VPNs with Pre-Shared Keys)