How to auto populate the email address in the survey question?

I'm not totally familiar with Eloqua, but to me it seems like it should be able to pipe in/merge fields from the contact list into the body of the email. You could

1) Make the anonymous link a Query String that contains the email associated with that record upon sending (this is where Eloqua would need to be able to merge the email address into the text of the email)
2) At the very start your Survey Flow, you will need to create an Embedded Data variable that matches the name you gave in the Query String to set the value of the variable in the survey to the email address in the link (just leave it as 'value will be set from panel or url')
3) You can pipe the embedded data value into the field where you want to collect email using piped text for the embedded variable name.

I would caution you to be very careful in your email that you don't promise this is anonymous. Although you are using the anonymous link, you are now connected part of their identity (email address) to their link individualizing it.

noopur_1791 as I mentioned I am not familiar with Eloqua. But the process I laid out above will achieve what you want but is contingent upon Eloqua being able to insert the recipient's email address into the body of the email (see step 1).

Without access to Eloqua I cannot test this theory for you. But this webpage makes it seem doable, see the section for "Add field merges to URL" https://docs.oracle.com/en/cloud/saas/marketing/eloqua-user/Help/EmailEditing/Tasks/AddingFieldMergesToEmails.htm