External threats to a computer or a computer network refer to potential risks or dangers that come from outside sources. These threats can compromise the security and integrity of the system, leading to unauthorized access, data breaches, or other malicious activities. Among the options provided, option A, "Ignorance," is not an external threat to a computer or a computer network. Here's an explanation of why:

1. Ignorance:
Ignorance refers to a lack of knowledge or awareness about computer security practices. While it is important for users to be educated about safe computing habits, ignorance itself is not an external threat. Rather, it can contribute to vulnerabilities and increase the risk of falling victim to external threats.

2. Trojan Horses:
Trojan horses are a type of malware that disguise themselves as legitimate software or files. They are often downloaded or executed unknowingly by users and can perform malicious actions, such as stealing sensitive information or providing unauthorized access to the system. Trojan horses are indeed external threats as they are designed and distributed by attackers.

3. Adware:
Adware is a type of software that displays unwanted advertisements on a user's computer. It is typically bundled with freeware or shareware programs and can track user behavior and collect personal information. Adware can be considered an external threat as it infiltrates the system through external sources and compromises user privacy.

4. Crackers:
Crackers, also known as malicious hackers or black hat hackers, are individuals who break into computer systems with malicious intent. They exploit vulnerabilities in software or network security to gain unauthorized access, steal information, or disrupt system operations. Crackers are external threats as they actively target and attack computer networks from outside sources.

In conclusion, while ignorance can contribute to vulnerabilities and increase the risk of falling victim to external threats, it is not an external threat itself. The other options provided, including Trojan horses, adware, and crackers, are all external threats that pose risks to computer systems and networks.

A: Ignorance
- Ignorance is not an external threat to a computer or computer network. It refers to a lack of knowledge or awareness about computer security practices, but it is not a direct threat itself.
B: Trojan horses
- Trojan horses are a type of malware that disguises itself as legitimate software to gain unauthorized access or control over a computer or network. They can cause significant damage by stealing sensitive information, modifying data, or allowing remote control by hackers.
C: Adware
- Adware is a type of software that displays unwanted advertisements on a user's computer. While it can be annoying and intrusive, it is not typically considered a severe external threat to the overall security of a computer or network.
D: Crackers
- Crackers, also known as malicious hackers, are individuals who exploit vulnerabilities in computer systems or networks for personal gain or malicious intent. They can engage in activities such as unauthorized access, data theft, or disruption of services.

Answer: A: Ignorance