The Avesta / əˈvɛstə / is the primary collection of religious texts of Zoroastrianism, composed in the otherwise unrecorded Avestan language.

Answer this question based on the passage: zend avesta is the holy book of which religion? Let's have some stream of consciousness first.
The important part for answering this question is: The Avesta / əˈvɛstə / is the primary collection of religious texts of Zoroastrianism, composed in the otherwise unrecorded Avestan language. So, the answer is Zoroastrianism.