Duralumin is an alloy of:
(A) Al, Mg and Li
(B) Al, Cu, Mg and Mn
(C) Al, Fe, Ca and Ni
 (D) Al, Ni and Ag

Hint: It is the trade name for the earliest types of age hardenable aluminium alloys . This alloy is lightweight, used in aircraft construction and is also resistant to corrosion.
Step by step answer:
-First of all let us see what an alloy is. An alloy is a blend of metals or we can say that a combination of more than one metal forms an alloy.
Example: white gold (mixture of gold and silver), brass, bronze, steel (mixture of iron, carbon, nickel, etc), duralumin, amalgams, etc.
-It was discovered by a German metallurgist named Alfred Wilm. In 1903 he by chance noticed that if an alloy of aluminium with 4% copper is kept at room temperature for some days, it hardens. This gave him the idea of this alloy.
-Duralumin basically refers to an alloy of aluminium and some other elements. It slowly hardens with passing time.
-Nowadays some copper-aluminium alloys are also known as duralumin.
-As the name suggests the main component of duralumin is aluminium, and it is mixed with copper, manganese and magnesium (which causes its hardness with time).
  Composition of various components: Aluminium = 95 %
                                                               Copper = 4 %
                                                                Magnesium = 0.5 %
                                                                 Manganese = 0.5 %
So, the correct option is: (B) Al, Cu, Mg and Mn.
Additional information: Duralumin is made of more than 90% aluminium which makes it lightweight and so it is preferred in aircraft construction, aircraft fittings as well as space booster tankage.

Note: Another alloy of aluminium called alloy 7075 is also used to strengthen aluminium aircraft structures. It is composed of 5.6% zinc, 2.5% magnesium, 1.6 % copper and a very small amount of Si, Fe, Mn, Ti, etc. both duralumin and alloy 7075 have common use in aircrafts. Do not confuse them with each other, they are 2 different alloys.