A diatomic molecule is:
A)Brass
B)Graphite
C)Silicon
D)nitrogen

Hint: To answer this question, you should recall the concept of diatomic entities. The word di means two hence, the term diatomic will refer to molecules containing two atoms.

Complete Step by step solution:
We know from the definition of diatomic, it refers to the molecules which contain two atoms which may be the same or different. Diatomic molecules with the same atoms are known as homonuclear, like:\[{H_2}\] or\[{O_2}\]. If any diatomic molecule consists of two different atoms it is known as heteronuclear like $CO$or$NO$. Now let us analyse each option systematically.
Brass is an alloy and is not a molecule. This option is wrong and can be eliminated
Graphite is an allotrope of carbon. This option is wrong and can be eliminated
Silicon is a single atom of an element. This option is wrong and can be eliminated
Nitrogen is a diatomic molecule which exists as ${N_2}$ a gas. This is the correct option.
Therefore, we can conclude that the correct answer to this question is option D.

Additional information: In heteronuclear diatomic molecules, atomic orbitals only mix when the electronegativity values are similar. While the molecular orbitals of homonuclear species contain equal contributions from each interacting atomic orbital, molecular orbitals of heteronuclear species contain different atomic orbital contributions. In heteronuclear diatomic species orbital interactions that produce bonding or antibonding orbitals occur if there is sufficient overlap between atomic orbitals, as determined by their symmetries and similarity in orbital energies.

Note: The most important diatomic elements you need to remember are the elements ending with "-gen" including halogens and hydrogen. To remember their names mnemonic for the diatomic elements is: Have No Fear of Ice-Cold Beer which corresponds to Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine and Bromine.