Answer:
Upward delivery or downward displacement of air
Step-by-step explanation:
The method of collection of a gas is dependent on it's density.
Gases that are less dense than air are collected by upward delivery or downward displacement of air while gases that are denser than air are collected by downward delivery or upward displacement of air.
Hydrogen is less dense than air hence it is collected by upward delivery or downward displacement of air.