Mi hermano ya había el libro para la escuela. a. leía b. leído c. leyendo

This sentence is written in Preterite pluperfect tense, also called pluperfect or Past Perfect which is used to talk about actions that were completed before some point in the past.

This verbal tense has the following structure:

Now, taking into account that:

-In spanish the verbs must be conjugated according the subject and must match in genre and number.  

-The subject of the sentence is the third person in singular él (he), which is mi hermano (my brother)

The correct conjugation of the verb leer (to read) is leído:

Therefore the complete sentence is:

My brother had already read the book for school