The correct answer is D. Cryptography.
Cryptography is a method of storing and transmitting data in a particular coded form so that only those can read and process it for whom it is intended. It includes encoding and decoding of data.
Blockchain is a system of recording information in a way that makes it difficult or impossible to change, hack, or cheat the system.
Programming is a process that involves writing instructions for a computer to follow.
Cloud computing is the on-demand delivery of IT resources over the Internet with pay-as-you-go pricing.