The shape of carbocation is:

Step 1: Analyzing Carbocation Structure Carbocations are species where a carbon atom has six electrons in its valence shell and bears a positive charge.

• The carbon is $sp^2$ hybridized.
• It has three $\sigma$ bonds and one empty $2p$ orbital perpendicular to the plane of the bonds.

Step 2: Determining the Shape The $sp^2$ hybridization results in a trigonal planar geometry with bond angles of approximately $120^\circ$.

$\boxed{\text{Answer: (a) trigonal planar}}$