According to Huckel’s rule, a compound is said to be aromatic, if it contains:
a. - (4n+2) $\pi $ bonds
b. - (4n+2) $\sigma $ bonds
c. - (4n+2) carbon atoms
d.- (4n+2) $\pi $ electrons

Hint: In organic compounds, Hückel's rule helps in deciding whether a planar ring molecule will have aromatic properties or not.

Complete answer:
Aromatic compounds are more stable than theoretically predicted as for simple alkenes; the additional stability is due to the delocalized cloud of electrons, called resonance energy.
Properties of simple aromatic compound are:
1.-The molecule must have $4n + 2$ electrons in a conjugated system of p orbitals,on \[s{p^2}\]-hybridized atoms ( i.e. huckel’s rule )
2.-The molecule must be (close to) planar (implicit in the requirement for conjugation).
3.-The molecule must be cyclic.
4.-The molecule must have a continuous ring of p atomic orbitals (there cannot be any \[\;s{p^3}\;\] atoms in the ring).
Let’s talk about Huckel’s $4n + 2$ Pi Electron Rule.A ring-shaped cyclic molecule is said to follow the Huckel rule when the total number of pi electrons belonging to the molecule can be equated to the formula ($4n+2$) where n can be any integer with a positive value (including zero).
For example, molecules following Huckel’s rule have only been established for values of ‘n’ ranging from zero to six.
The total number of pi electrons in the benzene molecule depicted below can be found to be 6, obeying the $4n+2$ $\pi $ electron rule where $n=1$.

Thus, the benzene molecule is an aromatic compound as it obeys the Huckel rule.
In such way Huckel’s rule is used for identifying aromatic compounds, having ($4n+2$) $\pi $ electrons (values of ‘n’ ranging from zero to six)

Hence the correct option is (d) ($4n+2$) $\pi $ electrons.

Note:
Don’t get confused with ($4n+2$) $\pi $ electrons and ($4n+2$) $\pi $bonds. Both are different things. No. of electrons is always double then the number of bonds.