Gated ion channels i. exist because active transporters cannot maintain the gradient at a rate equal to non-gated ion flow. ii. remain open unless acted on by an external stimulus. iii. remain open to all ions that are smaller than the pore diameter. iv. can transport varying ions.

IV. Can transport varying ions.

Explanation:

Gated ion channels intervene in the transit of ions through the cell membrane. The transport through these channels is extremely fast, and they are highly selective since their narrow pores limit the passage to those ions with an appropriate size and charge. This way, specific gated ion channels allow the transit of Na⁺, K⁺, Ca²⁺, and Cl⁻ through the membrane. Also, most of the ion channels are not permanently open, but they open transiently in response to specific stimuli (a ligand or a change in voltage).

1 is the answer to this problem