The best answer that best explains the term identity theft based on the options provided for this question is when someone uses your private information, pretends to be you in order to obtain things, money, or services for his/herself.
Identity theft simply refers to a deliberate attempt by someone to use someone else's personal information for certain fraudulent activities. Most times, those who engage in this act see it as a way of gaining financial advantage and other benefits. At times, someone whose identity has been stolen or assumed by someone else will pay greatly for whatever fraudulent acts his identify has been used for.
This means if you happen to become a victim, you might end up paying for what you know nothing about. Information such as your date of birth, full name, Gmail account, bank account number, driver's license number, etc. can be used by fraudsters to defraud you and others.