Answer: (a) Sodium and potassium; (b) Sonority; (c) Graphite
(a) Sodium (Na) and potassium (K) are soft metals that can be cut with a knife.
(b) Metals produce a ringing sound when struck; this property is called sonority (metals are sonorous).
(c) Graphite, an allotrope of carbon, is a non-metal that conducts electricity well.