The correct answer is Happily.

“Happily” is an adverb that modifies the adjective “happy.” It describes how someone is feeling or acting. The other options provided are not adverbs.

“Happier” is a comparative adjective used to compare two levels of happiness.
“Happiest” is a superlative adjective used to describe the highest level of happiness.
“Happiness” is a noun that represents the state or feeling of being happy.