I gave him ______ books I had fill the blank with correct determiners.
Sabina, before giving the answer, I would first explain you what are determiners.Determiners are the words that are prefixed or that come before a noun in a sentence to determine its' meaning. They can be articles - a, an,the; demonstratives- this, that, these, those; possessives - my, your, our, his, her, its, their etc.
So, the answer to your query is "the" which makes the sentence:-
I gave him the books I had.
Hope this helps.