Acluster of atoms whose magnetic fields are aligned in the same direction is a magnetic

A cluster of atoms whose magnetic fields are aligned in the same direction is a magnetic DIPOLE

Explanation:

When all the atoms are randomly arranged in a given material then in that case the magnetic field due to individual atom is randomly aligned and the vector sum of all the field will be zero

Now when we put the material in external magnetic field then all the atoms will experience torque due to which these all have tendency to align themselves into same direction.

This alignment of all the atoms into the same direction is known as magnetization

Here in this magnetization all the atoms will setup into same direction and the dipole moment of each atom will add to give net magnetic dipole moment

So correct answer would be

A cluster of atoms whose magnetic fields are aligned in the same direction is a magnetic DIPOLE

c) a core temperature of only ten thousand kelvin