Choose the malicious code which can distribute itself without using having to attach to a host file.

A computer worm is a standalone malicious program designed to replicate itself and spread to other computer systems, typically across a network. Its defining characteristic, which distinguishes it from a virus, is its ability to propagate without requiring a host file or human intervention. Worms exploit vulnerabilities in operating systems or applications to travel from one machine to another. This self-sufficiency in distribution and lack of dependency on a host program directly matches the criteria specified in the question.

A. A virus. A virus, by definition, requires a host file (like an executable or a document macro) to attach itself to for replication and propagation.
B. A logic bomb. A logic bomb is a malicious payload that activates when specific conditions are met; it is not self-propagating and does not distribute itself.
D. A Trojan horse. A Trojan horse is non-replicating malware disguised as legitimate software that relies on social engineering for distribution, not autonomous self-propagation.

1. Stallings, W., & Brown, L. (2018). Computer Security: Principles and Practice (4th ed.). Pearson. In Chapter 6, "Malicious Software," the text distinguishes between viruses and worms: "A worm is a program that can replicate itself and send copies from computer to computer across network connections. Upon arrival, the worm may be activated to replicate and propagate again. ... A computer worm does not require a host program." (p. 184).

2. MIT OpenCourseWare. (2014). 6.858 Computer Systems Security, Lecture 1: Introduction and Threat Models. Massachusetts Institute of Technology. The lecture notes state, "Worm: self-propagating program. Spreads over network... Unlike a virus, a worm can spread by itself." (p. 15). Available at: https://ocw.mit.edu/courses/6-858-computer-systems-security-fall-2014/resources/mit6858f14lec1/

3. Weaver, N., Paxson, V., Staniford, S., & Cunningham, R. (2003). A Taxonomy of Computer Worms. WORM '03: Proceedings of the 2003 ACM workshop on Rapid malcode. The introduction states: "Unlike viruses, worms are self-contained programs that propagate to other vulnerable computers; this gives them the potential to spread explosively." (p. 11). DOI: https://doi.org/10.1145/948187.948190