The air in flask Y gains heat from the hotter 90C water in Basin B and expands. The air then moves into the glass tube and pushes the drop of red ink towards Flask X.
Option 1 : Gains most heat from B, expands the most, pushes red ink furthest to X.
Option 2: Similar to Option 1, but gains less heat since water is only 30C, so it moves less towards X.
Option 3: Both sides same temp, expansion is the same, ink drop will not move.
Option 4: Same mechanism as Option 1, but in opposite direction