To help prevent snooping for someone using their laptop in busy airports, a screen-darkening filter would be the most relevant option. This filter helps obscure the screen's visibility from side angles, making it harder for people nearby to see sensitive information.
While dual-factor authentication and a strong password are essential for securing accounts and protecting data, they do not specifically address the issue of physical observation of the laptop screen in busy public spaces. Storing data in the cloud can also be secure, but it doesn't prevent someone from seeing what's displayed on your screen.
Therefore, the best choice here is a screen-darkening filter.