Which of the following cannot act as a ligand?

[NH₃—CH₂—CH₂—NH₃]²⁺

The correct answer is option 3. \([H_3N–CH_2–CH_2–NH_3]^{2+}\).

The structure, \([H_3N–CH_2–CH_2–NH_3]^{2+}\), does not have any atoms with lone pairs of electrons. The molecule is a polyatomic ion with a positive charge. The positive charge is due to the two nitrogen atoms donating their lone pairs of electrons to form a double bond with each other. This leaves the nitrogen atoms with no lone pairs of electrons. The hydrogen atoms also do not have any lone pairs of electrons, as they are bonded to the nitrogen atoms. Therefore, the structure, [\([H_3N–CH_2–CH_2–NH_3]^{2+}\), does not have any atoms with lone pairs of electrons, so it cannot act as a ligand.