Which flammable materials can spontaneously ignite in air when finely divided?

The correct choice identifies magnesium and lithium as flammable materials that can spontaneously ignite in air when finely divided. This phenomenon typically occurs due to the large surface area presented by finely divided particles, which increases their reactivity with oxygen in the air.

Magnesium, when in a fine powder form, can ignite in air at relatively low temperatures. Its high reactivity means that it can catch fire spontaneously if the conditions are right, particularly when it is finely divided. Similarly, lithium is known to react vigorously with water and can also catch fire when exposed to air, especially in its powdered form.

The other materials listed do not share this specific characteristic to the same extent. Aluminum and iron can create dust that is flammable under certain conditions, but they typically require a higher temperature to ignite compared to magnesium and lithium in fine form. Copper and selenium, as well as silver and lead, do not exhibit spontaneous ignition characteristics in air when finely divided, making magnesium and lithium the correct answer in this context.