A bubble rise in water, oil, or other liquid. What do you expect to see?

Question

accelerates constantly.
quickly reaches a constant speed.
speed zero.
acceleration is zero.

Answer 1

If it's deep enough so there's a significant pressure change as it climbs, it will expand visibly as it rises. Its speed should increase since its bouyancy increases by a larger factor in a given time than its surface area. At a shallow release depth the pressure is near atmospheric so the effect won't be as great. But even if it doesn't start at a great depth, it will grow a little, and if it's a bubble from a saturated gas solution (e.g., CO2 from a carbonated drink) it will accumulate more gas on its way up.
Lots of factors can change this. A very viscous liquid will have a greater drag, so the acceleration may be lass apparent. The bubble could be steam, piped into cool water. It could return to the liquid state before it reaches the surface. The bubble could ascend through layers of different temperature liquid, whhich could expand or shrink the bubble. Etc.