1. We need the species in which carbon has a sextet of electrons and acts as an electrophile.

2. Let us examine each option:

   A. Pentavalent carbon
   This is not the usual electron-deficient electrophilic carbon species referred to here. Hence, incorrect.

   B. Carbon free radical
   A free radical carbon has 7 electrons around it (three bonds and one unpaired electron), not a sextet. Hence, incorrect.

   C. Carbanion
   A carbanion has a lone pair on carbon and usually an octet of electrons. It is nucleophilic, not electrophilic. Hence, incorrect.

   D. Carbocation
   A carbocation has only 6 electrons around carbon (three bonds, empty $p$-orbital). Because it is electron-deficient, it acts as an electrophile. Hence, correct.

3. Therefore, the species having carbon with a sextet of electrons and acting as an electrophile is:

$\boxed{\text{Carbocation}}$