The production of a W boson in association with a single charm quark is studied using 4.6 fb⁻¹ of pp collision data at√s = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96₋{-0.30}^{+0.26} at Q² = 1.9 GeV², which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio σ(W⁺ +c̅) / σ(W⁻ +c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s-s̅ quark asymmetry.