Concept:An ammeter for an a.c. circuit must work on an effect that does not depend on the direction of current, such as the heating effect.Explanation:In an a.c. circuit, the current changes direction periodically.A hot wire ammeter measures current based on the heating effect, which depends on I2R and therefore works for both a.c. and d.c.Moving coil instruments and tangent galvanometers rely on the direction of current and work only with d.c.Answer:A. Hot wire ammeter