To answer this question, the user needs to have knowledge of different religious texts and their associated religions. In this case, the question is asking for the holy book of the Parsis, so the user needs to be familiar with the religious beliefs and practices of the Parsi community.

Now, let's go through each option and explain why it is right or wrong:

A. Zend Avesta: This option is correct. The Zend Avesta is the holy book of the Parsis. It contains the sacred texts of Zoroastrianism, the religion followed by the Parsis.

B. Koran: This option is incorrect. The Koran is the holy book of Islam, not the Parsis.

C. Bible: This option is incorrect. The Bible is the holy book of Christianity, not the Parsis.

D. Guru Granth Sahib: This option is incorrect. The Guru Granth Sahib is the holy book of Sikhism, not the Parsis.

Therefore, the correct answer is:

A. Zend Avesta