The correct answer is B. Cryptography.
Cryptography is the study of secret writing. It is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). Cryptography is used in a wide variety of applications, including computer security, data protection, and information assurance.
Decryption is the process of converting encrypted text back into its original form. It is the opposite of encryption.
A firewall is a security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It is used to protect computer systems and networks from unauthorized access, use, disclosure, disruption, modification, or destruction.
The study of encryption is not called firewall.