To choose the correct option we are to analyse the forces acting on the bubble .
Let the volume of the bubble at the certain instant be
The mass of the gaseous material in the bubble be
The downward gravitational pull on it will be
The upward buoyant force on it at the inatant considered will be
We know that
So net force acting at that instant
Hence the positive acceleration of the bubble at that instant will be
So option (a) is correct.