Bad Qubits — Learn Quantum by Doing

Multiple choice

In Kitaev's exactly solvable chain (fine-tuned topological limit: chemical potential mu=0, hopping = pairing = t), the two unpaired Majorana modes left at the ends of the wire are combined into a single nonlocal fermionic mode f. Why can no local perturbation read or flip the qubit stored in its occupation n_f?

Because Majorana operators do not satisfy any anticommutation relations
Because the two end Majoranas have infinite energy and cannot be excited
Because the qubit is encoded redundantly across all N sites of the chain at once
Because f is built from Majorana operators located at opposite ends of the wire, so the protection is geometric