The C1 transit visa is required for individuals who intend to temporarily pass through the US as they are in transit to another country. You are required to get an individual C1 transit visa regardless of your age (meaning even infants or the elderly have to apply for an individual C1 transit visa too.) The C1 transit visa is considered a form of non immigrant visa.
One way to legally enter the US even without a C1 transit visa is to apply for acceptance in the Visa Waiver Program. Without the Visa Waiver Program in force, you will not be permitted to enter the US without a C1 transit visa.
One of the reasons for C1 transit visa rejection is if you lack sufficient travel documents upon arrival in the US. If you fail to pay your visa fees that may qualify as one of the reasons for C1 transit visa rejection as well. You should also show evidence that you are simply passing through the US, such as when you’re taking a connecting flight to a third country at a US airport, to prove that your final destination is not the US. If you failed to prove that you have sufficient funds in your name to make it to your final destination (that is not the US.) You might also be asked to provide other documents by the consular officer, due to special circumstances such as being HIV positive.