= Bell twirling followed by triplet symmetrization
{c}
Average the four correlated local Pauli operations to remove coherences in the <Bell states> basis. A one-party Pauli operation can permute the largest of the four resulting weights into the singlet sector; that weight is at least $1/4$. Correlated common local unitaries then permute the three triplet projectors, while <collective-unitary covariance of the two-qubit singlet> keeps the singlet projector unchanged. Averaging all six permutations makes the triplet weights equal, producing a <two-qubit Werner state>. Shared randomness or <local operations and classical communication> coordinates these local operations. Common unitaries alone cannot increase an initially small singlet weight, because that weight is invariant under them.
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