Which type of malware blocks access to a system until a ransom is paid?

Ransomware is specifically designed to block access to a system or data until a ransom is paid by the victim. This form of malware typically encrypts files or locks users out of their systems, displaying a message that demands payment to restore access. The primary objective of ransomware is financial gain through coercion, making it distinct from other types of malware.

In contrast, spyware aims to gather information about a user or organization without their knowledge, typically for the purpose of data theft. Adware, on the other hand, generates unwanted advertisements, which can be intrusive but do not generally restrict access to the system. Trojan horses disguise themselves as legitimate software but can serve various malicious purposes, including opening backdoors for further attacks, but they do not inherently block access like ransomware does.

Therefore, the unique characteristic of ransomware, which is to hold a system hostage until payment is made, clearly establishes it as the correct answer.