To solve this question, the user needs to know about common web application vulnerabilities and their characteristics.

The correct answer is: C. Session fixation

Option A, SQL Injection, is incorrect because it is a type of attack that targets the database layer of a web application, not the HTTP protocol layer that handles form submissions.

Option B, Cross Site Request Forgery (CSRF), is incorrect because it involves tricking a user into submitting a form on a different website that performs an action on the user's behalf on a target website, not manipulating the values submitted within a form on the same website.

Option D, Session Hijacking, is incorrect because it involves stealing or guessing a user's session ID to impersonate them on a web application, not manipulating the values submitted within a form.

Option C, Session Fixation, is the correct answer. Session fixation is a type of attack where an attacker sets a user's session ID before the user logs in, then waits for the user to authenticate with that ID. Once the user logs in, the attacker can use the known session ID to access the user's session and perform actions on their behalf. In this case, the attacker could set a cookie value in the HTML form submission and then use that value to access the user's session.