This is the 744th most frequent Japanese word.
美味しく
"Deliciously" or "tastily."
In this sentence, "美味しく" (oishiku) functions as an adverb describing the verb "する" (to make/to do), indicating the desired state transformation.
スープをもっと美味しくする方法を教えてください。
Please tell me a way to make the soup more delicious.
Here, "美味しく" (oishiku) is used adverbially with the verb "溶ける" (to melt), describing how the action is taking place.
アイスクリームが美味しく溶けています。
The ice cream is melting deliciously.
Here, "美味しく" (oishiku) is an adverb modifying the verb "作る" (to make), specifying the manner in which the action is performed.