Dashiel is 20 months old. his mother tells him that he must not touch the candle that she just lit, and then she turns her back to continue tidying the room. when she turns back around, she sees dashiel trying to dip his finger in the melted wax under the flame. what best explains dashiel's behavior?

The answer which best explains Dashiel's behavior is that Dashiel's actions were driven by curiosity.
His mother told him not to touch the candle, but as he still doesn't understand why that is dangerous, he also couldn't understand why she forbade him from touching it. Thus, he was curious to see what would happen, which is why he wanted to touch it even if he didn't before.