What is the best item to use for water purification if iodine tablets are not available?

The most effective method for water purification among the options given, when iodine tablets are unavailable, is to boil water. Boiling is a reliable way to kill pathogens, viruses, and microorganisms present in the water and significantly reduces the risk of waterborne diseases. Water should be boiled for at least one minute (or longer at higher altitudes) to ensure that it is safe for drinking.

While charcoal has some ability to filter out certain impurities, it is not as effective as boiling when it comes to ensuring the elimination of harmful microorganisms. Hydrogen peroxide can disinfect water but is not typically used for purification on its own and can be harmful if ingested in large quantities. Iodine crystals would provide a similar function to iodine tablets and are not suitable in this context since the question assumes they are not available.

Therefore, boiling water stands out as the best method for purifying water in the absence of iodine tablets, as it is a straightforward and widely recommended technique that ensures safe drinking water.