Answer: we can complete the sentence given by the question as:
"When the candle was lit the wax melted because the molecules abosrbed / received energy and moved faster."
Step-by-step explanation:
The question requires us to complete the sentence given, explaining in terms of heat and temperature what happens to the wax when the candle is lit.
When a candle is lit, the wax near the wick (usually a piece of cotton that holds the flame) is melted by action of the transfer of heat from the flame. The wax, which is in solid state, melts because its molecules absorb the energy that is transferred in form of heat and start moving faster. The melted wax also moves, by capilarity action, towards the wick, and it "suffers" the action of the heat from the flame again.
Considering the information above, we can complete the sentence given by the question as:
"When the candle was lit the wax melted because the molecules abosrbed / received energy and moved faster."