(A): Tropylium cation is aromatic in nature

(R): The only property that determines its aromatic behaviour is its planar structure.

Tropylium cation has ${\mathrm{sp}}^2$carbons in its ring. It also has 6$\mathrm{\pi }$ electrons, participating in delocalization and follows (4n+2)$\mathrm{\pi }$ electrons rule(Huckel's rule. It is a planar molecule. Thus, all these four factors are essential for a compound to be aromatic. Hence, the assertion(A) is true and the Reason(R) is false.