It should be unsmudged left thumb impression .

Have a look at NEET 2020 Admit Card - Salient Features

• NTA has provided a mandatory COVID-19 undertaking with the NEET admit card. This is a mandatory document and must be filled by the candidates indicating whether they have flue symptoms or have been in contact with a COVID +ve patient.
• Before leaving for the NEET exam, all candidates have to affix their photograph in the space provided, place an unsmudged left thumb impression in the given space on the undertaking.
• Their parents must affix their signature in the NEET self declaration form given with the admit card. Candidates must however NOT put their signature in the NEET admit card 2020. This must be placed in front of the invigilator.
• Contact-free frisking and checking of the NEET admit card will be enforced at the examination hall. The contact free checking of the admit card will be by using the bar code/ QR code given on the NEET admit card 2020.
• Candidates must leave their NEET 2020 admit card with the invigilator after completion of the exam and before leaving the exam hall else their NEET answer sheets will not be evaluated.

For more details,

https://medicine.careers360.com/articles/neet-admit-card

Thank You.