If the muḍāf noun كِتَابٌ is joined to the third-person masculine singular pronoun ـهُ in the genitive case, what is the correct written form?

Using Attached Pronouns as the Muḍāf Ilayh and Their Spelling Rules

• كِتَابِهِ
• كِتَابُهُ
• كِتَابَهُ