Predict which of the following systems would be aromatic and why ?

Explanation

Compound (i) contains (4n +2) `pi` electrons where n=2, i.e., it contains 5 x 2 =10 `pi`-electrons and hence according to Huckel rule , it is aromatic.
Compound (iii) is also aromatic since it contains two benzene rings each one of which contains (4n +2) `pi`-electrons with n=1, i.e., 3 x 2 =6 electrons
Compound (ii) is a `4 n pi` (n=1) electron system and hence is an antiaromatic compound.
Although compound (iv) contains 6 `pi`-electrons , i.e., (4n +2) `pi` electrons with n=1 , yet the system is not completely conjugated due to the presence of a `sp^3` -hybridized carbon. In other words, this compound is not planar and hence is not aromatic .