Identify the species

A carbocation is a positively charged carbon species, typically represented as $R_3C^+$.

Hybridisation of the positively charged carbon

In a carbocation, the central carbon forms three sigma bonds and has no lone pair.

So, the carbon uses $sp^2$ hybrid orbitals for bonding.

Geometry from $sp^2$ hybridisation

An $sp^2$ hybridised atom has three electron domains arranged in one plane with bond angles close to $120^\circ$.

Therefore, the geometry is: $\text{Trigonal planar}$

Check the options

• A: tetrahedral $\rightarrow$ incorrect, this is for $sp^3$ carbon with four bonded groups.
• B: diagonal pyramidal $\rightarrow$ incorrect.
• C: diagonal $\rightarrow$ incorrect.
• D: trigonal planar $\rightarrow$ correct.

Final answer

The shape of a carbocation is trigonal planar.