Among the options presented, which cannot serve as a valid proof of your identity?

When you need to prove who you are, official organizations need to be sure you are really you. That's why they ask for specific documents. Think about it: what makes a document official? Usually, it's something issued by a recognized authority, like the government, a bank, or a utility company. These documents link your name and address to an official record, making them reliable. A recent gas bill or a bank statement works because these companies verify your identity before sending you statements. A document from the Home Office, the government department dealing with immigration, is obviously a valid form of ID. However, a letter from a friend, while perhaps heartfelt, doesn't carry any official weight. Anyone could write a letter saying they know you, so it doesn't provide the necessary verification that official bodies require. It lacks that crucial link to a trusted, independent source.