Final answer:
The correct answer is spyware, which is a malicious software that captures sensitive information by recording keystrokes.
Step-by-step explanation:
The type of security threat that can be transferred through email and is used to gain sensitive information by recording the keystrokes of the email recipient is known as spyware. Spyware is a type of malicious software that, once installed on a computer, can monitor and collect data without the user's knowledge, including following their keystrokes to capture passwords, financial data, and other sensitive information. While all the options listed can pose serious security threats, spyware is specifically designed for the purpose of surreptitious surveillance and therefore is the correct answer to the question.