Which is used by GETVPN, FlexVPN and DMVPN?

NHRP is used by DMVPN and FlexVPN for dynamic spoke-to-spoke tunnel resolution, but GETVPN does not use NHRP because it relies on a key server and multicast distribution instead.

MPLS is a label-switching forwarding technology used for traffic engineering and is not an inherent component of GETVPN, FlexVPN, or DMVPN.

GRE is used by DMVPN and FlexVPN to encapsulate packets before IPsec encryption, but GETVPN encrypts native routed IP packets directly and does not use GRE tunnels.

ESP is the IPsec encapsulation protocol that encrypts and authenticates packet payloads, and it is the single element shared across all three VPN technologies. GETVPN uses ESP in transport mode to encrypt native IP packets without tunneling, while DMVPN and FlexVPN use ESP within their respective tunnel constructs. No other listed protocol is universally required by all three.