Zinc metal is commonly used in the laboratory to prepare hydrogen gas. It is important to choose the appropriate form of zinc for this purpose. The correct form of zinc to be used in this case is granules.

Granules:
- Zinc granules are small pieces of zinc metal that have a relatively large surface area compared to their volume.
- The large surface area allows for more efficient reactions, as it provides more contact points for the reactants.
- When zinc granules are reacted with an acid, such as hydrochloric acid (HCl), they undergo a chemical reaction that produces hydrogen gas (H2).
- This reaction is known as a single displacement reaction, where the zinc metal displaces the hydrogen from the acid.
- The hydrogen gas is released as a byproduct of the reaction and can be collected and used for various purposes.

Other forms of zinc:
While zinc can be found in other forms such as rods, powder, and filings, these forms are less suitable for the preparation of hydrogen gas due to their properties.

Rods:
- Zinc rods are solid cylindrical pieces of zinc metal.
- The surface area of the rods is relatively small compared to their volume, which makes them less effective for reactions.
- The reaction between zinc rods and an acid would be slower and less efficient in producing hydrogen gas.

Powder:
- Zinc powder consists of finely divided particles of zinc metal.
- While the powder form does have a larger surface area compared to rods, it is still not as effective as granules.
- The small particle size can lead to clumping and decreased reactivity, making it less suitable for the preparation of hydrogen gas.

Filings:
- Zinc filings are thin strips or shavings of zinc metal.
- Like powder, filings have a larger surface area compared to rods but are still not as efficient as granules.
- The thin and irregular shape of filings can lead to decreased reactivity and slower reaction rates.

In summary, the granular form of zinc is the most suitable for preparing hydrogen gas in the laboratory. Its larger surface area allows for more efficient reactions and faster production of hydrogen gas.