This is among the many mimetic native in the Japanese language, and also it is commonly used in a day-to-day life.

You can write this word in both Katakana ニコニコ and also in Hiragana にこにこ.

We don"t recognize the exact origin the this word, therefore it might be much better just to offer you the answer come this one!...

The prize is number 3 - laugh !