Which of the following is an aromatic species?

An aromatic species is cyclic, planar, fully conjugated and has $(4n+2)\pi$ electrons. The cyclopropenyl cation has 2 $\pi$ electrons ($n=0$). Cyclobutadiene (4) and the cyclopentadienyl cation (4) are antiaromatic, and cyclopentadiene has an sp3 carbon, so it is not fully conjugated.