Which metal crystallises in a simple cubic structure?
A. Polonium
B. Copper
C. Nickel
D. Iron

Hint:
Simple cubic structure is also called a primitive cubic system. Think about where the atoms are placed in a simple cubic unit. The answer is a radioactive element that does not have any stable isotopes due to high internal energy and atomic size.

Complete step by step solution:
There are three types of cubic structures: simple cubic, face centred, and body centred. They all have one factor in common that atoms are present at all the eight vertices of the cube. The face centred structure has atoms present at the centre of each face and the body centred structure has an atom at the centre of the cube. The simple cubic structure comprises only the atoms present at the eight vertices.
Copper and nickel crystalize in face centred lattices and iron crystallizes in a body centred lattice. We know that all of these metals have at least one non-radioactive, stable isotope.
All elements try to reduce the energy required for bonding by being as tightly packed as possible. Polonium also tries to do the same but due to radioactivity and a large atomic radius, this is not possible. Thus, it crystallizes in a simple cubic structure.
Hence, the correct option is ‘A. Polonium’.

Note:
Remember that polonium is isovalent with oxygen, this means that it has 6 electrons in its outermost orbit. It bonds along the ${{p}_{x}},\text{ }{{p}_{y}}\text{ and }{{p}_{z}}\text{ }$orbitals to form a simple cubic lattice. It is the most loosely packed lattice. It is common for metals to form face centred or body centred cubic lattices