The correct answer is option 1: the presence of sulphur in gunpowder decreases the ignition temperature.

Gunpowder, also known as black powder, is a mixture of sulfur, charcoal, and potassium nitrate. The sulfur in gunpowder serves as a fuel and also lowers the ignition temperature of the mixture.

Ignition temperature refers to the minimum temperature at which a substance can ignite or catch fire. By decreasing the ignition temperature, sulphur makes the gunpowder more easily ignitable. This is important for the effective and efficient functioning of firearms, where the ignition of gunpowder needs to occur quickly and reliably.

Options 2, 3, and 4 are incorrect explanations for the presence of sulfur in gunpowder. Sulfur does not directly impact the final temperature or explosiveness of gunpowder. Additionally, sulfur does not make gunpowder smokeless; the smoke generated by gunpowder comes from the combustion of the other components like charcoal and potassium nitrate.