Permalloy is primarily an iron-nickel magnetic alloy, typically consisting of about 80% nickel and 20% iron. This specific composition gives permalloy its characteristic high magnetic permeability and low coercivity, making it useful in applications like magnetic shielding and electrical transformers.

The correct option is A. Nickel.

B. Tin: Tin is a primary component in bronze (a copper-tin alloy) and in some types of solder, but not the main secondary constituent of permalloy.

C. Chromium: Chromium is a key element in stainless steel (an alloy of iron, chromium, and sometimes nickel) to provide corrosion resistance, not the primary alloying element with iron in permalloy.

D. Copper: Copper is the primary component in brass and bronze. While some specific permalloy variations may contain small amounts of copper, it is not the main constituent with iron in the general definition of permalloy.