See it as the inertia of the surrounding air is higher than the inertia of the helium due to its lower density.
That means when accelerating (forward) the air is flowing "backwards" faster than the helium.
Discussion
Important to note is that the air is in a container. Helium is less dense than the air, so buoyancy follows the direction of the force. So C would be acceleration forward. If it was something more dense than air, it would be A acceleration forward as there would always be displacement with acceleration.