D  

Rigorous hand hygiene by staff is the single most effective measure to halt patient-to-patient transmission of postoperative wound pathogens; studies in ICUs show that improving hand-washing compliance sharply reduces nosocomial infection rates, whereas environmental fumigation or blanket antibiotic prophylaxis offer far less impact [Doebbeling et al., 1992, PMID 1285746; Salama et al., 2013, PMID 23290090].