Answer: either form is fine.
Data is typically treated in contemporary English as a "mass noun", an undifferentiated collection (like "sand") that takes the singular "is".
This is the most common usage today.
But it CAN also be treated as a plural of "datum" and take "are" with equal ease and correctness.
If we recast the sentence with an explicit plural noun:
"In this section you can make a payment: verify carefully that all the answers submitted ARE correct and confirm by clicking the Proceed button."
then you must use "are".
If there was prior discussion of "data items" or the like, then the established context may cause the plural treatment to be preferred for continuity.