C. Replacing potentially harmful characters. The purpose of the escape function when securing HTML text data is to replace potentially harmful characters with their corresponding HTML escape sequences. This helps prevent issues such as cross-site scripting (XSS) by ensuring that user input or other dynamic content is properly sanitized before rendering in HTML to prevent the interpretation of special characters as code.

The correct answer is C. Replacing potentially harmful characters. Explanation: The escape function in the context of securing HTML text data is used to replace potentially harmful characters with safe representations. This helps prevent attacks such as Cross-Site Scripting (XSS), where an attacker injects malicious code into a website, which is then executed by unsuspecting users.

The purpose of the escape function in securing HTML text data is to replace potentially harmful characters with their corresponding HTML entities to prevent them from being interpreted as HTML or script code. Therefore, the correct answer is C.