Explanation

We use in after delight.

In is a preposition here.

Delight is a verb here.

Delight in by doing something کسی کام کو کرکے خوشی حاصل کرنا

He delights in playing with children وہ بچوں کے ساتھ کھیل کر خوشی حاصل کرتا ہے