The correct option is 1, which states that magnesium is used in fireworks to make a bright white light.

Option 2, which suggests that magnesium is used in gemstones in jewelry, is incorrect. Magnesium is not typically used in gemstones or jewelry; instead, gemstones are usually made from minerals such as diamonds, rubies, sapphires, and emeralds.

Option 3, which mentions the examination of the alimentary canal of a patient, is also incorrect. Magnesium is not used in the examination of the alimentary canal (digestive system) of a patient. Diagnostic procedures like endoscopy or colonoscopy may be used to examine the alimentary canal, but they do not involve the use of magnesium.

Option 4, which states that magnesium is used in the purification of water, is also incorrect. While Magnesium can be found in some water sources, it is not used specifically for the purpose of water purification. Water purification methods typically involve processes like filtration, chlorination, and reverse osmosis.

In conclusion, the correct answer is option 1 as magnesium is commonly used in fireworks to produce a bright white light.