Which types of vessels are categorized as "combatant" vessels of war?

Combatant vessels of war are specifically designed and equipped for engaging in military operations and combat. These types of vessels typically include those that possess weapon systems, which enable them to engage in offensive or defensive actions during conflicts.

Amphibious aircraft/landing craft carriers and destroyers are prime examples of combatant vessels. Amphibious carriers are designed to transport troops and equipment, enabling them to conduct operations such as land assaults, while destroyers are multi-role warships capable of anti-surface, anti-air, and anti-submarine warfare. Both types are integral to naval operations, serving critical functions in combat scenarios.

In contrast, cargo ships, oil tankers, research vessels, and survey ships are not designed for combat and do not have the necessary weaponry or defensive systems to be classified as combatants. While coastal patrol boats can engage in military functions, they are often considered smaller patrol craft rather than major combatant vessels like destroyers or amphibious carriers. Thus, the designation of "combatant" is aligned with vessels specifically built for warfare, marking option A as the correct choice.