Encapsulating Security Payload (ESP) is used within IPsec to provide confidentiality, along with integrity and authentication of the data. It encrypts the payload data and optional header to ensure that sensitive information remains confidential when sent over an insecure network. The other options do not specifically address data confidentiality. Authentication Header (AH) provides authenticity and integrity but does not encrypt data, thus does not guarantee confidentiality of the communication. Full-duplex transmission relates to the direction data can travel in a network and does not pertain to data confidentiality or encryption.