Mischmetal is

an alloy of lanthanoid metals

The correct answer is option 3. an alloy of lanthanoid metals.

Mischmetal is a rare-earth alloy that consists of about 50% cerium, 25% lanthanum, 15% neodymium, and 10% other rare-earth metals. It is also known as cerium mischmetal or rare-earth mischmetal.

Mischmetal is a hard, silvery-white metal that is used in a variety of applications, including lighter flints, spark plugs, and magnets.

The other options are incorrect.

Option (1): Mischmetal is not an alloy of aluminum.

Option (2): Mischmetal is not a mixture of chromium and PbCrO₄.

Option (4): Mischmetal is not an alloy of copper.