In Zeigler-Natta polymerization of ethylene, the active species is

Terminal 1-alkenes (ethylene and alkenes with the vinyl double bond) are polymerized as follows using Ziegler Natta catalysts: 

${nCH}_2=CHR\to {{[CH}_2-CHR]}_n$ .

Zeigler-Natta catalyst is formed by the action of (C₂​H₅​)₃​Al and TiCl_4​. Active species is Ti³⁺ as (C₂​H₅​)₃​Al) reduces TiCl₄​ to TiCl₃​.

Ti³⁺ has active site vacant and thus, can accommodate one alkyl group.

Hence the correct option is (D).