The correct answer is A. Mg.
The wire of flash bulbs is made of magnesium because it is a highly flammable metal that burns with a bright light. When the flash bulb is triggered, a small amount of electricity is passed through the magnesium wire, causing it to ignite and produce a bright flash of light.
Magnesium is a good choice for this application because it is relatively inexpensive, easy to work with, and has a high ignition temperature. It is also a relatively safe material to use, as it
does not produce toxic fumes when it burns.The other options are not as good choices for the wire of flash bulbs. Barium is a heavy metal that is not flammable. Copper is a good conductor of electricity,
but it does not burn with a bright light. Silver is a precious metal that is too expensive to use in flash bulbs.In conclusion, the wire of flash bulbs is made of magnesium because it is a highly flammable metal that burns with a bright light.