The correct option is C Compound (b) is the least stable
For compound (a)
Cyclopentadiene:
Here one of the carbon atoms is sp3 hybridised. So, it is not planar. It is non-aromatic.
For compound (b)
Cyclopentadienyl cation:
All the carbon atoms are sp2 hybridised, and hence the structure is planar. It has conjugation and number of π e−=4
It follows (4n) π e− rule.
It is anti-aromatic.
For compound (c)
Cyclopentadienyl anion:
It is planar.
Has conjugation
Follows Huckel's rule (4n+2) e− , since it has 6 π e−
It is Aromatic.
Stability order :
Aromatic>Non-aromatic>Anti-aromatic
So, options (b) and (c) are correct