Which process of lifting is most significant in the following example?

Question

A man attached 45 helium-filled weather balloons to a lawn chair to go for a ride. He rose to an altitude of nearly 16,000 ft. The high elevations and lack of oxygen made him dizzy, so he shot out several balloons with a pellet gun and decended back to earth. Which process was most significant in the balloon ride? Density lifting, Orographic lifting, Convergence lifting, or Frontal lifting? and Why?

Answer 1

Density lifting.

The difference between the densities of the air in the balloons (high pressure) and the air outside them (lower) caused the balloons to rise.