Which of these items requires close inspection when considering dangerous goods?

The toolbox is the correct choice for close inspection when considering dangerous goods because it can contain a variety of items that may be classified as hazardous. Toolboxes often hold tools and materials that could potentially be dangerous, such as sharp instruments or substances that might pose a chemical risk. Inspecting a toolbox allows authorities to assess whether any items inside could violate safety regulations or pose a risk to the aircraft, passengers, or crew.

In contrast, while the other items may require some level of inspection, they typically do not warrant the same level of scrutiny concerning dangerous goods. A sealed cardboard box could contain a variety of items but is not inherently indicative of dangerous goods unless further inspection reveals otherwise. An empty suitcase generally does not raise suspicions about hazardous materials, and a backpack might occasionally hold items that need checking, but toolboxes are specifically associated with potentially dangerous tools and materials, justifying their need for closer examination.