Right answer: leído
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:
Subject + had + main verb in past participle + complement
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:
había leído
Therefore the complete sentence is:
Mi hermano ya había leído el libro para la escuela
My brother had already read the book for school
