One chain of alternating nags and nams is connected to another chain via

Peptidoglycan is a polymer consisting of sugars and amino acids that form a mesh-like layer outside the plasma membrane of most  bacteria. The sugar components consists of alternating residues of beta-(1-4) linked N-acetylglucosamine (NAG) and N- acetylmuramic acid (NAM). One chain of alternating of alternating NAGs and NAMs is connected to another chain via tetrapeptides composed of four amino acids and make up the "peptido" portion of peptidoglycan.

option a is correct i.e.

only sodium ions can pass through the membrane.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.