Concept:The correct word must describe skill or expertise in a subject, especially for a certificate.Explanation:A certificate is usually awarded for a high level of ability or skill."Proficiency" means a high degree of skill or expertise."Efficacy" means effectiveness of a method or drug."Efficiency" means working well without waste."Sufficiency" means being enough or adequate.Only "proficiency" fits the context of a certificate in English.Answer:C. proficiency