You had better sleep early. [Syntax]

The correct answer is "ほうが" (hou ga). This sentence uses the advice pattern "V-たほうがいいです" (You had better V). "早(はや)く寝(ね)た" is the past plain form of "早(はや)く寝(ね)る" (to sleep early). Adding "ほうが" completes the grammatical structure, giving the advice "You had better sleep early". "君(きみ)は" is the topic. "早(はや)く" is an adverb, and "寝(ね)た" is the verb in its -ta form, both already present. "いいです" means "it is good" and follows "ほうが" to complete the phrase.

正解は「ほうが」です。「V-たほうがいいです」は「Vした方がよい」というアドバイスの表現です。「早く寝た」は動詞「寝る」の「た形」なので「寝たほうがいい」と続きます。「早く」と「寝た」はすでに文中にあり、「いいです」は「ほうが」の後ろにくる述語のため不適当です。