Most stable species:

Step 1: Identifying Aromatic Stability

• (a) Cyclopropenyl cation: $2\pi$ electrons, $n=0$ in $4n+2$. It is Aromatic and highly stable.
• (b) Cyclopropenyl anion: $4\pi$ electrons. Anti-aromatic. (Unstable).
• (c) Cyclopentadienyl cation: $4\pi$ electrons. Anti-aromatic. (Unstable).
• (d) Cyclohexadienyl radical/cation: Non-aromatic.

Step 2: Conclusion The cyclopropenyl cation (a) is the only aromatic species among the choices.

$\boxed{\text{Answer: (a) Cyclopropyl cation}}$