Final answer:
A Known-plaintext attack is when an attacker has access to both the plaintext and ciphertext and aims to determine the encryption key or compromise the cryptographic algorithm.
Step-by-step explanation:
A type of attack on cryptosystems where both the plaintext and ciphertext are known to the attacker is called a Known-plaintext attack. In such attacks, the adversary uses this knowledge to uncover the encryption key or to develop a more intricate attack against the cryptographic algorithm being used. The scenario described fits option B, and thus the complete sentence is: A Known-plaintext attack is one in which you know both the plaintext and the ciphertext, where the most obvious goal is to find the decryption key.